Soluble support material for electrophotography-based additive manufacturing

ABSTRACT

A support material for printing a support structure with an electrophotography-based additive manufacturing system, the support material including a composition having a charge control agent and a thermoplastic copolymer having aromatic groups, (meth)acrylate-based ester groups, carboxylic acid groups, and anhydride groups, with a high anhydride conversion. The composition is provided in a powder form having a controlled particle size, and the support material is configured for use in the electrophotography-based additive manufacturing system having a layer transfusion assembly for printing the support structure in a layer-by-layer manner, and is at least partially soluble in an aqueous solution.

CROSS-REFERENCE TO RELATED APPLICATIONS

This Application is a Continuation Application of U.S. patentapplication Ser. No. 13/944,478, filed Jul. 17, 2013; the contents ofwhich are incorporated by reference.

BACKGROUND

The present disclosure relates to additive manufacturing systems forprinting three-dimensional (3D) parts and support structures. Inparticular, the present disclosure relates to consumable materials forprinting 3D parts and support structures using an imaging process, suchas electrophotography.

Additive manufacturing systems are used to build 3D parts from digitalrepresentations of the 3D parts (e.g., AMF and STL format files) usingone or more additive manufacturing techniques. Examples of commerciallyavailable additive manufacturing techniques include extrusion-basedtechniques, ink jetting, selective laser sintering, powder/binderjetting, electron-beam melting, and stereolithographic processes. Foreach of these techniques, the digital representation of the 3D part isinitially sliced into multiple horizontal layers. For each sliced layer,a tool path is then generated, which provides instructions for theparticular additive manufacturing system to form the given layer.

For example, in an extrusion-based additive manufacturing system, a 3Dpart or model may be printed from a digital representation of the 3Dpart in a layer-by-layer manner by extruding a flowable part material.The part material is extruded through an extrusion tip carried by aprint head of the system, and is deposited as a sequence of roads on asubstrate in an x-y plane. The extruded part material fuses topreviously deposited part material, and solidifies upon a drop intemperature. The position of the print head relative to the substrate isthen incremented along a z-axis (perpendicular to the x-y plane), andthe process is then repeated to form a 3D part resembling the digitalrepresentation.

In fabricating 3D parts by depositing layers of a part material,supporting layers or structures are typically built underneathoverhanging portions or in cavities of objects under construction, whichare not supported by the part material itself. A support structure maybe built utilizing the same deposition techniques by which the partmaterial is deposited. The host computer generates additional geometryacting as a support structure for the overhanging or free-space segmentsof the 3D part being formed, and in some cases, for the sidewalls of the3D part being formed. The support material adheres to the part materialduring fabrication, and is removable from the completed 3D part when theprinting process is complete.

In two-dimensional (2D) printing, electrophotography (i.e., xerography)is a technology for creating 2D images on planar substrates, such asprinting paper and transparent substrates. Electrophotography systemstypically include a conductive support drum coated with aphotoconductive material layer, where latent electrostatic images areformed by electrostatic charging, followed by image-wise exposure of thephotoconductive layer by an optical source. The latent electrostaticimages are then moved to a developing station where toner is applied tocharged areas, or alternatively to discharged areas of thephotoconductive insulator to form visible images. The formed tonerimages are then transferred to substrates (e.g., printing paper) andaffixed to the substrates with heat and/or pressure.

SUMMARY

An aspect of the present disclosure is directed to a support materialfor printing a support structure with an electrophotography-basedadditive manufacturing system. The support material includes acomposition having a charge control agent and a copolymer with aromaticgroups, (meth)acrylate-based ester groups, carboxylic acid groups, andanhydride groups, with a high anhydride conversion. The composition isprovided in a powder form having a controlled particle size (e.g., a D50particle size ranging from about 5 micrometers to about 30 micrometers),and the support material is configured for use in theelectrophotography-based additive manufacturing system having a layertransfusion assembly for printing the support structures in alayer-by-layer manner, preferably in coordination with the printing of3D parts, is at least partially soluble in an aqueous solution (e.g., anaqueous alkaline solution).

In some embodiments, the above-discussed support material may beprovided in an interchangeable cartridge or other similar device, alongwith carrier particles, for use with the electrophotography-basedadditive manufacturing systems.

Another aspect of the present disclosure is directed to a method formanufacturing a support material for use in an electrophotography-basedadditive manufacturing system. The method includes polymerizing monomerscomprising one or more ethylenically-unsaturated aromatic monomers, oneor more alkyl (meth)acrylate monomers, and one or more (meth)acrylicacid monomers, to produce a thermoplastic copolymer in acid form, whichcomprises aromatic groups, (meth)acrylate-based ester groups, andcarboxylic acid groups. The method also includes heating thethermoplastic copolymer in acid form to convert a portion of thecarboxylic acid groups to anhydride groups with an anhydride conversionthat is at least 90% of a maximum anhydride conversion for thethermoplastic copolymer, thereby producing a thermoplastic copolymer inanhydride form. The method further includes blending a charge controlagent to the thermoplastic copolymer in anhydride form, and forming theblend into a powder form having a D50 particle size ranging from about 5micrometers to about 30 micrometers.

Another aspect of the present disclosure is directed to a method forprinting a support structure with an electrophotography-based additivemanufacturing system having an electrophotography engine, a transfermedium, and a layer transfusion assembly. The method includes providinga support material to the electrophotography-based additivemanufacturing system, where the support material compositionallyincludes a charge control agent and a thermoplastic copolymer havingaromatic groups, (meth)acrylate-based ester groups, carboxylic acidgroups, and anhydride groups, and with a high anhydride conversion, andwhere the support material is provided in a powder form.

The method also includes triboelectrically charging the support materialto a desired triboelectric charge (e.g., a Q/M ratio having a negativecharge or a positive charge, and a magnitude ranging from about 5micro-Coulombs/gram to about 50 micro-Coulombs/gram), and developinglayers of the support structure from the charged support material withthe electrophotography engine. The method further includeselectrostatically attracting the developed layers from theelectrophotography engine to the transfer medium, moving the attractedlayers to the layer transfusion assembly with the transfer medium, andtransfusing the moved layers to previously-printed layers of the supportstructure with the layer transfusion assembly.

DEFINITIONS

Unless otherwise specified, the following terms as used herein have themeanings provided below:

The term “copolymer” refers to a polymer having two or more monomerspecies, and includes terpolymers (i.e., copolymers having three monomerspecies).

The terms “preferred” and “preferably” refer to embodiments of theinvention that may afford certain benefits, under certain circumstances.However, other embodiments may also be preferred, under the same orother circumstances. Furthermore, the recitation of one or morepreferred embodiments does not imply that other embodiments are notuseful, and is not intended to exclude other embodiments from theinventive scope of the present disclosure.

Reference to “a” chemical compound refers one or more molecules of thechemical compound, rather than being limited to a single molecule of thechemical compound. Furthermore, the one or more molecules may or may notbe identical, so long as they fall under the category of the chemicalcompound. Thus, for example, “a” styrene-butyl acrylate-methacrylic acidcopolymer is interpreted to include one or more polymer molecules of thecopolymer, where the polymer molecules may or may not be identical(e.g., different molecular weights and/or isomers).

The terms “at least one” and “one or more of” an element are usedinterchangeably, and have the same meaning that includes a singleelement and a plurality of the elements, and may also be represented bythe suffix “(s)” at the end of the element. For example, “at least onecopolymer”, “one or more copolymers”, and “copolymer(s)” may be usedinterchangeably and have the same meaning.

The term “(meth)acrylate” includes acrylate, methacrylate, and/or acombination thereof. Similarly, the term “(meth)acrylic acid” includesacrylic acid, methacrylic acid, and/or a combination thereof.

Directional orientations such as “above”, “below”, “top”, “bottom”, andthe like are made with reference to a direction along a printing axis ofa 3D part. In the embodiments in which the printing axis is a verticalz-axis, the layer-printing direction is the upward direction along thevertical z-axis. In these embodiments, the terms “above”, “below”,“top”, “bottom”, and the like are based on the vertical z-axis. However,in embodiments in which the layers of 3D parts are printed along adifferent axis, the terms “above”, “below”, “top”, “bottom”, and thelike are relative to the given axis.

The term “providing”, such as for “providing a material” and the like,when recited in the claims, is not intended to require any particulardelivery or receipt of the provided item. Rather, the term “providing”is merely used to recite items that will be referred to in subsequentelements of the claim(s), for purposes of clarity and ease ofreadability.

Unless otherwise specified, temperatures referred to herein are based onatmospheric pressure (i.e. one atmosphere).

The terms “about” and “substantially” are used herein with respect tomeasurable values and ranges due to expected variations known to thoseskilled in the art (e.g., limitations and variabilities inmeasurements).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front view of an example electrophotography-based additivemanufacturing system for printing 3D parts and support structures frompart and support materials of the present disclosure.

FIG. 2 is a schematic front view of a pair of electrophotography enginesof the system for developing layers of the part and support materials.

FIG. 3 is a schematic front view of an alternative electrophotographyengine, which includes an intermediary drum or belt.

FIG. 4 is a schematic front view of a layer transfusion assembly of thesystem for performing layer transfusion steps with the developed layers.

FIG. 5 is an example proton NMR spectrum of an example support materialcopolymer in acid form.

FIG. 6 is a plot of dynamic viscosity versus temperature for examplesupport material copolymers, illustrating how molecular weight andanhydride conversion affects melt rheology behaviors of the copolymers.

FIG. 7 is a plot of dynamic viscosity versus temperature for additionalexample support material copolymers produced in larger-scale batches,illustrating melt rheology behaviors of the copolymers.

FIG. 8 is a plot of dynamic viscosity versus temperature for additionalexample support material copolymers with heat absorbers blended atdifferent temperatures, illustrating melt rheology behaviors of thecopolymers.

FIG. 9 is a plot of powder particle size distributions for examplesupport materials of the present disclosure.

DETAILED DESCRIPTION

The present disclosure is directed to a sacrificial soluble supportmaterial, which is molecularly engineered for use in anelectrophotography-based additive manufacturing system to print supportstructures, in association with a part material used to print 3D parts,with high resolutions and fast printing rates. During a printingoperation, electrophotography (EP) engines may develop or otherwiseimage each layer of the part and support materials using theelectrophotographic process. The developed layers are then transferredto a layer transfusion assembly where they are transfused (e.g., usingheat and/or pressure) to print one or more 3D parts and supportstructures in a layer-by-layer manner.

In comparison to 2D printing, in which developed toner particles can beelectrostatically transferred to printing paper by placing an electricalpotential through the printing paper, the multiple printed layers in a3D environment effectively prevents the electrostatic transfer of partand support materials after a given number of layers are printed (e.g.,about 15 layers). Instead, each layer may be heated to an elevatedtransfer temperature, and then pressed against a previously-printedlayer (or to a build platform) to transfuse the layers together in atransfusion step. This allows numerous layers of 3D parts and supportstructures to be built vertically, beyond what is otherwise achievablevia electrostatic transfers.

As discussed below, the support material of the present disclosure is apowder-based, soluble support material that is engineered for use inelectrophotography-based additive manufacturing system, and tocomplement a suitable part material (e.g., thermoplastic-based partmaterials), such as an acrylonitrile-butadiene-styrene (ABS) partmaterial. The support material is particularly suitable for use with theABS part material disclosed in co-filed U.S. patent application Ser. No.13/944,472, entitled “ABS Part Material For Electrophotography-BasedAdditive Manufacturing”, the disclosure of which is incorporated byreference to the extent that it does not conflict with the presentdisclosure.

The support material functions as sacrificial material for the partmaterial, and is desirable in cases where overhanging features arerequired in the final 3D part structure, where significant angularslopes exist in the 3D part, where it is essential to also preservedelicate features in the 3D part, such as small orifices or controlledpore structures, and in some situations, to laterally encase the 3Dpart. Once the 3D part has been printed, the support structure of thesacrificial support material may be removed to reveal the completed 3Dpart, preferably without damaging any of the critical or delicategeometrical features of the 3D part. To accomplish this, the supportmaterial is soluble in an aqueous solution, such as an aqueous alkalinesolution, allowing the support structure to be dissolved away from the3D part.

These requirements, however, have imparted significant challenges inproducing a support material that is suitable for use in anelectrophotography-based additive manufacturing system. For example, asdiscussed below, each layer of the support material is preferablytransfused along with an associated layer of the part material. As such,the support material also preferably has thermal properties (e.g., aglass transition temperature) and a melt rheology that are similar to,or more preferably, substantially the same as the thermal properties andmelt rheology of its associated part material.

Moreover, the support material is preferably capable of accepting andmaintaining a stable triboelectric charge that is similar to, or morepreferably, substantially the same as that of the associated partmaterial to allow the part and support materials to be transferred tothe layer transfusion assembly together. Furthermore, the supportmaterial is preferably capable of being produced in a powder-form usinga cost-efficient processing technique, preferably exhibits good adhesionto the part material, and is preferably thermally stable during a layertransfusion process.

Accordingly, the support material of the present disclosure has beendeveloped to balance these competing factors. Briefly, the supportmaterial compositionally includes a thermoplastic copolymer, a chargecontrol agent, and optionally, a heat absorber and/or one or moreadditional materials, such as a flow control agent. The thermoplasticcopolymer includes aromatic groups, (meth)acrylate-based ester groups,carboxylic acid groups, and anhydride groups, where a ratio of theanhydride groups-to-carboxylic acid groups is preferably maximized orotherwise increased in favor of anhydride conversion for the copolymer.

A preferred thermoplastic copolymer for the support material is aterpolymer derived from one or more ethylenically-unsaturated aromaticmonomers (e.g., styrene), one or more alkyl (meth)acrylate monomers(e.g., butyl acrylate), and one or more (meth)acrylic acid monomers(e.g., methacrylic acid), where carboxylic acid groups from the one ormore (meth)acrylic acid monomers are preferably convertedintra-molecularly to anhydride groups to the fullest extent possible.

It has been found that if the anhydride conversion is not substantiallyor fully maximized, the carboxylic acid groups of the support materialcan undergo further anhydride conversion during the layer transfusionstep, which can result in foaming and bubbling. The foaming and bubblingcan accordingly result in a loss of support structure and part quality.This is in comparison to extrusion-based additive manufacturingtechniques incorporating support materials of soluble thermoplasticcopolymers, which typically have substantially lower anhydrideconversions. Furthermore, the support material also preferably has goodadhesion to the part material layers to effectively support the partmaterial layers during the printing operation, while also being solublein an aqueous-based solution for sacrificial removal in a post-printingremoval step.

FIGS. 1-4 illustrate system 10, which is an exampleelectrophotography-based additive manufacturing system for printing 3Dparts from a part material (e.g., an ABS part material), and associatedsupport structures from the support material of the present disclosure.As shown in FIG. 1, system 10 includes a pair of EP engines 12 p and 12s, belt transfer assembly 14, biasing mechanisms 16 and 18, and layertransfusion assembly 20. Examples of suitable components and functionaloperations for system 10 include those disclosed in Hanson et al., U.S.Publication Nos. 2013/0077996 and 2013/0077997, and in Comb et al., U.S.patent application Ser. Nos. 13/790,382 and 13/790,406.

EP engines 12 p and 12 s are imaging engines for respectively imaging orotherwise developing layers of the part and support materials, where thepart and support materials are each preferably engineered for use withthe particular architecture of EP engine 12 p or 12 s. As discussedbelow, the imaged layers may then be transferred to belt transferassembly 14 (or other transfer medium) with biasing mechanisms 16 and18, and carried to layer transfusion assembly 20 to print the 3D partsand associated support structures in a layer-by-layer manner.

In the shown embodiment, belt transfer assembly 14 includes transferbelt 22, belt drive mechanisms 24, belt drag mechanisms 26, loop limitsensors 28, idler rollers 30, and belt cleaner 32, which are configuredto maintain tension on belt 22 while belt 22 rotates in the rotationaldirection of arrows 34. In particular, belt drive mechanisms 24 engageand drive belt 22, and belt drag mechanisms 26 may function as brakes toprovide a service loop design for protecting belt 22 against tensionstress, based on monitored readings via loop limit sensors 28.

System 10 also includes controller 36, which is one or more controlcircuits, microprocessor-based engine control systems, and/ordigitally-controlled raster imaging processor systems, and which isconfigured to operate the components of system 10 in a synchronizedmanner based on printing instructions received from host computer 38.Host computer 38 is one or more computer-based systems configured tocommunicate with controller 36 to provide the print instructions (andother operating information). For example, host computer 38 may transferinformation to controller 36 that relates to the sliced layers of the 3Dparts and support structures, thereby allowing system 10 to print the 3Dparts and support structures in a layer-by-layer manner.

The components of system 10 may be retained by one or more framestructures, such as frame 40. Additionally, the components of system 10are preferably retained within an enclosable housing (not shown) thatprevents ambient light from being transmitted to the components ofsystem 10 during operation.

FIG. 2 illustrates EP engines 12 p and 12 s, where EP engine 12 s (i.e.,the upstream EP engine relative to the rotational direction of belt 22)develops layers of the support material, and EP engine 12 p (i.e., thedownstream EP engine relative to the rotational direction of belt 22)develops layers of the part material. In alternative embodiments, thearrangement of EP engines 12 p and 12 s may be reversed such that EPengine 12 p is upstream from EP engine 12 s relative to the rotationaldirection of belt 22. In further alternative embodiments, system 10 mayinclude three or more EP engines for printing layers of additionalmaterials.

In the shown embodiment, EP engines 12 p and 12 s may include the samecomponents, such as photoconductor drum 42 having conductive drum body44 and photoconductive surface 46. Conductive drum body 44 is anelectrically-conductive drum (e.g., fabricated from copper, aluminum,tin, or the like) that is electrically grounded and configured to rotatearound shaft 48. Shaft 48 is correspondingly connected to drive motor50, which is configured to rotate shaft 48 (and photoconductor drum 42)in the direction of arrow 52 at a constant rate.

Photoconductive surface 46 is a thin film extending around thecircumferential surface of conductive drum body 44, and is preferablyderived from one or more photoconductive materials, such as amorphoussilicon, selenium, zinc oxide, organic materials, and the like. Asdiscussed below, surface 46 is configured to receive latent-chargedimages of the sliced layers of a 3D part or support structure (ornegative images), and to attract charged particles of the part orsupport material of the present disclosure to the charged or dischargedimage areas, thereby creating the layers of the 3D part or supportstructure.

As further shown, EP engines 12 p and 12 s also includes charge inducer54, imager 56, development station 58, cleaning station 60, anddischarge device 62, each of which may be in signal communication withcontroller 36. Charge inducer 54, imager 56, development station 58,cleaning station 60, and discharge device 62 accordingly define animage-forming assembly for surface 46 while drive motor 50 and shaft 48rotate photoconductor drum 42 in the direction of arrow 52.

In the shown example, the image-forming assembly for surface 46 of EPengine 12 s is used to form layers 64 s of the support material(referred to as support material 66 s), where a supply of supportmaterial 66 s may be retained by development station 58 (of EP engine 12s) along with carrier particles. Similarly, the image-forming assemblyfor surface 46 of EP engine 12 p is used to form layers 64 p of the partmaterial (referred to as part material 66 p), where a supply of partmaterial 66 p may be retained by development station 58 (of EP engine 12p) along with carrier particles.

Charge inducer 54 is configured to generate a uniform electrostaticcharge on surface 46 as surface 46 rotates in the direction of arrow 52past charge inducer 54. Suitable devices for charge inducer 54 includecorotrons, scorotrons, charging rollers, and other electrostaticcharging devices.

Imager 56 is a digitally-controlled, pixel-wise light exposure apparatusconfigured to selectively emit electromagnetic radiation toward theuniform electrostatic charge on surface 46 as surface 46 rotates in thedirection of arrow 52 past imager 56. The selective exposure of theelectromagnetic radiation to surface 46 is directed by controller 36,and causes discrete pixel-wise locations of the electrostatic charge tobe removed (i.e., discharged to ground), thereby forming latent imagecharge patterns on surface 46.

Suitable devices for imager 56 include scanning laser (e.g., gas orsolid state lasers) light sources, light emitting diode (LED) arrayexposure devices, and other exposure device conventionally used in 2Delectrophotography systems. In alternative embodiments, suitable devicesfor charge inducer 54 and imager 56 include ion-deposition systemsconfigured to selectively directly deposit charged ions or electrons tosurface 46 to form the latent image charge pattern. As such, as usedherein, the term “electrophotography” includes ionography.

Each development station 58 is an electrostatic and magnetic developmentstation or cartridge that retains the supply of part material 66 p orsupport material 66 s, preferably in powder form, along with carrierparticles. Development stations 58 may function in a similar manner tosingle or dual component development systems and toner cartridges usedin 2D electrophotography systems. For example, each development station58 may include an enclosure for retaining the part material 66 p orsupport material 66 s and carrier particles. When agitated, the carrierparticles generate triboelectric charges to attract the powders of thepart material 66 p or support material 66 s, which charges the attractedpowders to a desired sign and magnitude, as discussed below.

Each development station 58 may also include one or more devices fortransferring the charged part material 66 p or support material 66 s tosurface 46, such as conveyors, fur brushes, paddle wheels, rollers,and/or magnetic brushes. For instance, as surface 46 (containing thelatent charged image) rotates from imager 56 to development station 58in the direction of arrow 52, the charged part material 66 p or supportmaterial 66 s is attracted to the appropriately charged regions of thelatent image on surface 46, utilizing either charged area development ordischarged area development (depending on the electrophotography modebeing utilized). This creates successive layers 64 p or 64 s asphotoconductor drum 12 continues to rotate in the direction of arrow 52,where the successive layers 64 p or 64 s correspond to the successivesliced layers of the digital representation of the 3D part or supportstructure.

The successive layers 64 p or 64 s are then rotated with surface 46 inthe direction of arrow 52 to a transfer region in which layers 64 p or64 s are successively transferred from photoconductor drum 42 to belt22, as discussed below. While illustrated as a direct engagement betweenphotoconductor drum 42 and belt 22, in some preferred embodiments, EPengines 12 p and 12 s may also include intermediary transfer drumsand/or belts, as discussed further below.

After a given layer 64 p or 64 s is transferred from photoconductor drum42 to belt 22 (or an intermediary transfer drum or belt), drive motor 50and shaft 48 continue to rotate photoconductor drum 42 in the directionof arrow 52 such that the region of surface 46 that previously held thelayer 64 p or 64 s passes cleaning station 60. Cleaning station 60 is astation configured to remove any residual, non-transferred portions ofpart or support material 66 p or 66 s. Suitable devices for cleaningstation 60 include blade cleaners, brush cleaners, electrostaticcleaners, vacuum-based cleaners, and combinations thereof.

After passing cleaning station 60, surface 46 continues to rotate in thedirection of arrow 52 such that the cleaned regions of surface 46 passdischarge device 62 to remove any residual electrostatic charge onsurface 46, prior to starting the next cycle. Suitable devices fordischarge device 62 include optical systems, high-voltagealternating-current corotrons and/or scorotrons, one or more rotatingdielectric rollers having conductive cores with applied high-voltagealternating-current, and combinations thereof.

Transfer belt 22 is a transfer medium for transferring the developedsuccessive layers 64 p and 64 s from photoconductor drum 42 (or anintermediary transfer drum or belt) to layer transfusion assembly 16.Examples of suitable transfer belts for belt 22 include those disclosedin Comb et al., U.S. patent application Ser. Nos. 13/790,382 and13/790,406. Belt 22 includes front surface 22 a and rear surface 22 b,where front surface 22 a faces surfaces 46 of photoconductor drums 42and rear surface 22 b is in contact with biasing mechanisms 16 and 18.

Biasing mechanisms 16 and 18 are configured to induce electricalpotentials through belt 22 to electrostatically attract layers 64 p and64 s from EP engines 12 p and 12 s to belt 22. Because layers 64 p and64 s are each only a single layer increment in thickness at this pointin the process, electrostatic attraction is suitable for transferringlayers 64 p and 64 s from EP engines 12 p and 12 s to belt 22.

Controller 36 preferably rotates photoconductor drums 36 of EP engines12 p and 12 s at the same rotational rates that are synchronized withthe line speed of belt 22 and/or with any intermediary transfer drums orbelts. This allows system 10 to develop and transfer layers 64 p and 66s in coordination with each other from separate developer images. Inparticular, as shown, each part layer 64 p may be transferred to belt 22with proper registration with each support layer 64 s to preferablyproduce a combined part and support material layer 64. As discussedbelow, this allows layers 64 p and 64 s to be transfused together,requiring the part and support materials to have thermal properties andmelt rheologies that are similar or substantially the same. As can beappreciated, some layers transferred to layer transfusion assembly 20may only include support material 66 s or may only include part material66 p, depending on the particular support structure and 3D partgeometries and layer slicing.

In an alternative and less-preferred embodiment, part layers 64 p andsupport layers 64 s may optionally be developed and transferred alongbelt 22 separately, such as with alternating layers 64 p and 64 s. Thesesuccessive, alternating layers 64 p and 64 s may then be transferred tolayer transfusion assembly 20, where they may be transfused separatelyto print the 3D part and support structure.

In some preferred embodiments, one or both of EP engines 12 p and 12 smay also include one or more intermediary transfer drums and/or beltsbetween photoconductor drum 42 and belt 22. For example, as shown inFIG. 3, EP engine 12 p may also include intermediary drum 42 a thatrotates an opposing rotational direction from arrow 52, as illustratedby arrow 52 a, under the rotational power of motor 50 a. Intermediarydrum 42 a engages with photoconductor drum 42 to receive the developedlayers 64 p from photoconductor drum 42, and then carries the receiveddeveloped layers 64 p and transfers them to belt 22.

EP engine 12 s may include the same arrangement of intermediary drum 42a for carrying the developed layers 64 s from photoconductor drum 42 tobelt 22. The use of such intermediary transfer drums or belts for EPengines 12 p and 12 s can be beneficial for thermally isolatingphotoconductor drum 42 from belt 22, if desired.

FIG. 4 illustrates an example embodiment for layer transfusion assembly20. As shown, layer transfusion assembly 20 includes build platform 68,nip roller 70, heaters 72 and 74, post-fuse heater 76, and air jets 78(or other cooling units). Build platform 68 is a platform assembly orplaten of system 10 that is configured to receive the heated combinedlayers 64 (or separate layers 64 p and 64 s) for printing a 3D part andsupport structure, referred to as 3D part 80 and support structure 82,in a layer-by-layer manner. In some embodiments, build platform 68 mayinclude removable film substrates (not shown) for receiving the printedlayers 64, where the removable film substrates may be restrained againstbuild platform using any suitable technique (e.g., vacuum drawing,removable adhesive, mechanical fastener, and the like).

Build platform 68 is supported by gantry 84, which is a gantry mechanismconfigured to move build platform 68 along the z-axis and the x-axis toproduce a reciprocating rectangular pattern, where the primary motion isback-and-forth along the x-axis (illustrated by broken lines 86. Gantry84 may be operated by motor 88 based on commands from controller 36,where motor 88 may be an electrical motor, a hydraulic system, apneumatic system, or the like.

In the shown embodiment, build platform 68 is heatable with heatingelement 90 (e.g., an electric heater). Heating element 90 is configuredto heat and maintain build platform 68 at an elevated temperature thatis greater than room temperature (25° C.), such as at a desired averagepart temperature of 3D part 80 and/or support structure 82, as discussedin Comb et al., U.S. patent application Ser. Nos. 13/790,382 and13/790,406. This allows build platform 68 to assist in maintaining 3Dpart 80 and/or support structure 82 at this average part temperature.

Nip roller 70 is an example heatable element or heatable layertransfusion element, which is configured to rotate around a fixed axiswith the movement of belt 22. In particular, nip roller 70 may rollagainst rear surface 22 b in the direction of arrow 92 while belt 22rotates in the direction of arrow 34. In the shown embodiment, niproller 70 is heatable with heating element 94 (e.g., an electricheater). Heating element 94 is configured to heat and maintain niproller 70 at an elevated temperature that is greater than roomtemperature (25° C.), such as at a desired transfer temperature forlayers 64.

Heater 72 is one or more heating devices (e.g., an infrared heaterand/or a heated air jet) configured to heat layers 64 to a temperaturenear an intended transfer temperature of the part and support materials,such as at least a fusion temperature of the part and support materials,preferably prior to reaching nip roller 70. Each layer 64 desirablypasses by (or through) heater 72 for a sufficient residence time to heatthe layer 64 to the intended transfer temperature. Heater 74 mayfunction in the same manner as heater 72, and heats the top surfaces of3D part 80 and support structure 82 to an elevated temperature, such asat the same transfer temperature as the heated layers 64 (or othersuitable elevated temperature).

As mentioned above, the support material 66 s used to print supportstructure 82 preferably has thermal properties (e.g., glass transitiontemperature) and a melt rheology that are similar to or substantiallythe same as the thermal properties and the melt rheology of the partmaterial 66 p used to print 3D part 80. This allows part and supportmaterials of layers 64 p and 64 s to be heated together with heater 74to substantially the same transfer temperature, and also allows the partand support materials at the top surfaces of 3D part 80 and supportstructure 82 to be heated together with heater 74 to substantially thesame temperature. Thus, the part layers 64 p and the support layers 64 smay be transfused together to the top surfaces of 3D part 80 and supportstructure 82 in a single transfusion step as combined layer 64. Thissingle transfusion step for transfusing the combined layer 64 is notbelieved to be feasible without matching the thermal properties and themelt rheologies of the part and support materials.

Post-fuse heater 76 is located downstream from nip roller 70 andupstream from air jets 78, and is configured to heat the transfusedlayers to an elevated temperature in the post-fuse or heat-setting step.Again, the similar thermal properties and melt rheologies of the partand support materials allow post-fuse heater 76 to post-heat the topsurfaces of 3D part 80 and support structure 82 together in a singlepost-fuse step.

Prior to printing 3D part 80 and support structure 82, build platform 68and nip roller 70 may be heated to their desired temperatures. Forexample, build platform 68 may be heated to the average part temperatureof 3D part 80 and support structure 82 (due to the close melt rheologiesof the part and support materials). In comparison, nip roller 70 may beheated to a desired transfer temperature for layers 64 (also due to thesimilar thermal properties and melt rheologies of the part and supportmaterials).

During the printing operation, belt 22 carries a layer 64 past heater72, which may heat the layer 64 and the associated region of belt 22 tothe transfer temperature. Suitable transfer temperatures for the partand support materials include temperatures that exceed the glasstransition temperatures of the part and support materials, which arepreferably similar or substantially the same, and where the part andsupport materials of layer 64 are softened but not melted (e.g., atemperature of ranging from about 140° C. to about 180° C. for an ABSpart material).

As further shown in FIG. 4, during operation, gantry 84 may move buildplatform 68 (with 3D part 80 and support structure 82) in areciprocating rectangular pattern 86. In particular, gantry 84 may movebuild platform 68 along the x-axis below, along, or through heater 74.Heater 74 heats the top surfaces of 3D part 80 and support structure 82to an elevated temperature, such as the transfer temperatures of thepart and support materials. As discussed in Comb et al., U.S. patentapplication Ser. Nos. 13/790,382 and 13/790,406, heaters 72 and 74 mayheat layers 64 and the top surfaces of 3D part 80 and support structure82 to about the same temperatures to provide a consistent transfusioninterface temperature. Alternatively, heaters 72 and 74 may heat layers64 and the top surfaces of 3D part 80 and support structure 82 todifferent temperatures to attain a desired transfusion interfacetemperature.

The continued rotation of belt 22 and the movement of build platform 68align the heated layer 64 with the heated top surfaces of 3D part 80 andsupport structure 82 with proper registration along the x-axis. Gantry84 may continue to move build platform 68 along the x-axis, at a ratethat is synchronized with the rotational rate of belt 22 in thedirection of arrow 34 (i.e., the same directions and speed). This causesrear surface 22 b of belt 22 to rotate around nip roller 70 to nip belt22 and the heated layer 64 against the top surfaces of 3D part 80 andsupport structure 82. This presses the heated layer 64 between theheated top surfaces of 3D part 80 and support structure 82 at thelocation of nip roller 70, which at least partially transfuses heatedlayer 64 to the top layers of 3D part 80 and support structure 82.

As the transfused layer 64 passes the nip of nip roller 70, belt 22wraps around nip roller 70 to separate and disengage from build platform68. This assists in releasing the transfused layer 64 from belt 22,allowing the transfused layer 64 to remain adhered to 3D part 80 andsupport structure 82. Maintaining the transfusion interface temperatureat a transfer temperature that is higher than the glass transitiontemperatures of the part and support materials, but lower than theirfusion temperatures, allows the heated layer 64 to be hot enough toadhere to 3D part 80 and support structure 82, while also being coolenough to readily release from belt 22. Additionally, as discussedabove, the similar thermal properties and melt rheologies of the partand support materials allow them to be transfused in the same step.

After release, gantry 84 continues to move build platform 68 along thex-axis to post-fuse heater 76. At post-fuse heater 76, the top-mostlayers of 3D part 80 and support structure 82 (including the transfusedlayer 64) may then be heated to at least the fusion temperature of thepart and support materials in a post-fuse or heat-setting step. Thismelts the part and support materials of the transfused layer 64 to ahighly fusible state such that polymer molecules of the transfused layer64 quickly inter-diffuse to achieve a high level of interfacialentanglement with 3D part 80 and support structure 82.

Additionally, as gantry 84 continues to move build platform 68 along thex-axis past post-fuse heater 76 to air jets 78, air jets 78 blow coolingair towards the top layers of 3D part 80 and support structure 82. Thisactively cools the transfused layer 64 down to the average parttemperature, as discussed in Comb et al., U.S. patent application Ser.Nos. 13/790,382 and 13/790,406.

To assist in keeping 3D part 80 and support structure 82 at the averagepart temperature, in some preferred embodiments, heater 74 and/orpost-heater 76 may operate to heat only the top-most layers of 3D part80 and support structure 82. For example, in embodiments in whichheaters 72, 74, and 76 are configured to emit infrared radiation, 3Dpart 80 and support structure 82 may include heat absorbers and/or othercolorants configured to restrict penetration of the infrared wavelengthsto within the top-most layers. Alternatively, heaters 72, 74, and 76 maybe configured to blow heated air across the top surfaces of 3D part 80and support structure 82. In either case, limiting the thermalpenetration into 3D part 80 and support structure 82 allows the top-mostlayers to be sufficiently transfused, while also reducing the amount ofcooling required to keep 3D part 80 and support structure 82 at theaverage part temperature.

Gantry 84 may then actuate build platform 68 downward, and move buildplatform 68 back along the x-axis to a starting position along thex-axis, following the reciprocating rectangular pattern 86. Buildplatform 68 desirably reaches the starting position for properregistration with the next layer 64. In some embodiments, gantry 84 mayalso actuate build platform 68 and 3D part 80/support structure 82upward for proper registration with the next layer 64. The same processmay then be repeated for each remaining layer 64 of 3D part 80 andsupport structure 82.

In some preferred embodiments, a resulting 3D part 80 is encasedlaterally (i.e., horizontally to the build plane) in the supportstructure 82, such as shown in FIG. 4. This is believed to provide gooddimensional integrity and surface quality for the 3D part 80 while usinga reciprocating build platen 68 and a nip roller 70.

After the printing operation is completed, the resulting 3D part 80 andsupport structure 82 may be removed from system 10 and undergo one ormore post-printing operations. For example, support structure 82 derivedfrom the support material of the present disclosure may be sacrificiallyremoved from 3D part 80, such as by using an aqueous-based solution(e.g., an aqueous alkali solution). Under this preferred solubletechnique, support structure 82 may at least partially dissolve in thesolution, separating it from 3D part 80 in a hands-free manner.

In comparison, part materials such as an ABS part material arechemically resistant to aqueous alkali solutions. This allows the use ofan aqueous alkali solution to be employed for removing the sacrificialsupport structure 82 without degrading the shape or quality of 3D part80. Examples of suitable systems and techniques for removing supportstructure 82 in this manner include those disclosed in Swanson et al.,U.S. Pat. No. 8,459,280; Hopkins et al., U.S. Pat. No. 8,246,888; andDunn et al., U.S. Publication No. 2011/0186081; each of which areincorporated by reference to the extent that they do not conflict withthe present disclosure.

Furthermore, after support structure 82 is removed, 3D part 80 mayundergo one or more additional post-printing processes, such as surfacetreatment processes. Examples of suitable surface treatment processesinclude those disclosed in Priedeman et al., U.S. Pat. No. 8,123,999;and in Zinniel, U.S. Publication No. 2008/0169585.

As briefly discussed above, the support material of the presentdisclosure includes a thermoplastic copolymer, a charge control agent,and optionally a heat absorber (e.g., an infrared absorber) and/or oneor more additional materials, such as a flow control agent. The supportmaterial has also been developed to balance competing factors for use inan electrophotography-based additive manufacturing system (e.g., system10), and to be soluble in an aqueous solution (e.g., an alkaline aqueoussolution) for removal from the 3D part.

The thermoplastic copolymer of the support material is polymerized frommonomers that preferably include one or more ethylenically-unsaturatedaromatic monomers (e.g., styrene), one or more alkyl (meth)acrylatemonomers, and one or more (meth)acrylic acid monomers, where carboxylicacid groups from the one or more (meth)acrylic acid monomers arepreferably converted to anhydride groups to the fullest extent possible.

Example ethylenically-unsaturated aromatic monomers have the followingstructure:

where, in some embodiments, the hydrogen atoms in Formula 1 may beindependently substituted with one or more optional low-atomic weightgroups, such as an alkyl or ether group having 1-3 carbon atoms.Furthermore, in some embodiments, the ethylenically-unsaturated groupand the aromatic group may be separated by an optional chain linkage,such as a hydrocarbon or ether linkage having 1-8 carbon atoms.

In some further embodiments, one or more hydrogen atoms of the aromaticgroup may be independently substituted with one or more optionallow-atomic weight groups, such as an alkyl or ether group having 1-3carbon atoms. More preferably, the aromatic monomer includes thestructure shown above in Formula 1, with the ethylenically-unsaturatedvinyl group extending directly from the aromatic group (i.e., styrene).The aromatic monomers raise the glass transition temperature of theresulting copolymer and provide polymer hardness.

Example alkyl (meth)acrylate monomers have the following structure:

where R₁ is a hydrocarbon chain having 1-8 carbon atoms (more preferably2-5 carbon atoms). R₂ is a hydrogen atom, or an alkyl or ether grouphaving 1-3 carbon atoms (more preferably a hydrogen atom). Furthermore,in some embodiments, the ethylenically-unsaturated group and thecarbonyl group may be separated by an optional chain linkage, such as ahydrocarbon or ether linkage having 1-8 carbon atoms.

More preferably, the alkyl (meth)acrylate monomer includes the structureshown above in Formula 2, with the ethylenically-unsaturated vinyl groupextending directly from the carbonyl group, and most preferably where R₁is a hydrocarbon chain having 2-5 carbon atoms, and R₂ is a hydrogenatom (e.g., an alkyl acrylate, such as butyl acrylate). The alkyl(meth)acrylate monomers tend to lower the glass transition temperatureof the resulting copolymer, but provide polymer softness and elasticity.

Example (meth)acrylic acid monomers have the following structure:

where R₃ is a hydrogen atom, or an alkyl or ether group having 1-3carbon atoms (more preferably an alkyl group, such as a methyl group).Furthermore, in some embodiments, the ethylenically-unsaturated groupand the carboxylic acid group may be separated by an optional chainlinkage, such as a hydrocarbon or ether linkage having 1-8 carbon atoms.In its simplest form, the term “carboxylic acid group” refers to a—C(O)OH carboxyl group.

More preferably, the (meth)acrylic acid monomer includes the structureshown above in Formula 3, with the ethylenically-unsaturated vinyl groupextending directly from the carboxylic acid group, and most preferablywhere R₃ is a methyl group (i.e., methacrylic acid). The (meth)acrylicacid monomers also raise the glass transition temperature of theresulting copolymer, and impart aqueous solubility to the resultingcopolymer.

The polymerization of the thermoplastic copolymer may be performed witha free-radical polymerization reaction of the monomers, using anysuitable polymerization initiator, such as dibenzoyl peroxide. Thepolymerization is preferably performed in solution, so as to ensure theformation of a random, completely amorphous, copolymer. For example, thepolymerization reaction may be performed by charging the monomercomponents, along with a suitable carrier solvent (e.g., ethanol) to areaction vessel. The reaction vessel is preferably purged with an inertgas (e.g., nitrogen) and heated to a reaction temperature (e.g., fromabout 80° C. to about 85° C.). The polymerization initiator may then beintroduced to the reaction vessel, preferably in separate sub-doses toprevent the exothermic polymerization reaction from raising the reactiontemperature to the point where excessive solvent boiling would occur.

For example, the polymerization initiator may be added in three separatesteps, allowing the polymerization reaction to proceed for a suitableduration (e.g., two hours) between each addition. In this embodiment,the first addition of the polymerization initiator preferably rangesfrom about 50% to about 70% by weight of a total weight of thepolymerization initiator added. Correspondingly, the second addition ofthe polymerization initiator preferably ranges from about 20% to about30% by weight of the total weight of the polymerization initiator added,and the third addition of the polymerization initiator preferably rangesfrom about 5% to about 25% by weight of a total weight of thepolymerization initiator added.

It has been found that adjusting the total amount of the polymerizationinitiator added, and the relative amounts in the separate additions, cancontrol the molecular weight of the resulting thermoplastic copolymer.Accordingly, in a preferred embodiment in which the support material isintended to complement an ABS part material, the first addition of thepolymerization initiator ranges from about 63% to about 67% by weight,the second addition of the polymerization initiator ranges from about23% to about 27% by weight, and the third addition of the polymerizationinitiator ranges from about 9% to about 13% by weight, relative to thetotal weight of the polymerization initiator added. This preferredembodiment for the polymerization initiator is even more preferablycombined with the monomers of styrene, butyl acrylate, and methacrylicacid.

After the polymerization is completed, the resulting viscous copolymersolution may be diluted with a solvent (e.g., ethanol) and allowed tocool to room temperature. Next, the copolymer solution may beprecipitated into a non-solvent (e.g., cyclohexane) under vigorousstirring, and the solvent may be separated from resulting thermoplasticcopolymer. The thermoplastic copolymer may then be dried under vacuum atan elevated temperature (e.g., 100° C.) for a suitable duration, andthen recovered.

This polymerization reaction produces the thermoplastic copolymer withthe monomer units having desired molar ratios of the monomers, and,preferably, a controlled molecular weight. The ethylenically-unsaturatedaromatic monomers used to produce the thermoplastic copolymer mayconstitute from about 25% to about 50% by weight, more preferably fromabout 30% to about 40% by weight, and even more preferably from about32% to about 36% by weight, based on an entire weight of monomers usedto produce the thermoplastic copolymer.

The alkyl (meth)acrylate monomers used to produce the thermoplasticcopolymer may constitute from about 15% to about 35% by weight, morepreferably from about 20% to about 30% by weight, and even morepreferably from about 22% to about 28% by weight, based on an entireweight of monomers used to produce the thermoplastic copolymer. The(meth)acrylic acid monomers used to produce the thermoplastic copolymermay constitute from about 30% to about 50% by weight, more preferablyfrom about 35% to about 45% by weight, and even more preferably fromabout 38% to about 43% by weight, based on an entire weight of monomersused to produce the thermoplastic copolymer.

In some embodiments, the monomers used to polymerize the thermoplasticcopolymer may include one or more additional monomer compounds thatpreferably do not significantly detract from the strength, chemical, orthermal properties of the thermoplastic copolymer. For example, thethermoplastic copolymer may include monomers that function as chainextending units (e.g., ethylene units) for the copolymer backbone.

Accordingly, the additional monomers may collectively constitute from 0%by weight to about 10% by weight, based on the entire weight of themonomers used to produce the thermoplastic copolymer. In someembodiment, the additional monomers may constitute from about 0.1% toabout 5% by weight, based on the entire weight of the monomers used toproduce the thermoplastic copolymer. The remainder of the monomers usedto polymerize the thermoplastic copolymer accordingly consist of theabove-discussed ethylenically-unsaturated aromatic monomers, the alkyl(meth)acrylate monomers, and the (meth)acrylic acid monomers.

In other preferred embodiments, the monomers used to polymerize thethermoplastic copolymer consist essentially or completely of theethylenically-unsaturated aromatic monomers, the alkyl (meth)acrylatemonomers, and the (meth)acrylic acid monomers. In more preferredembodiments, the monomers used to polymerize the soluble copolymerconsist essentially or completely of styrene, butyl acrylate, andmethacrylic acid.

The polymerized thermoplastic copolymer, as synthesized by free-radicalpolymerization reaction from the above-discussed monomers, results in ahigh yield of the resulting copolymer in very high conversion of themonomers-to-copolymer. Furthermore, the carboxylic acid groups from the(meth)acrylic acid monomers typically remain unaffected, and extend aspendant groups from the copolymer backbone. As such, the thermoplasticcopolymer at this stage may be referred to as being in the “acid form”.

However, this acid form of the thermoplastic copolymer is not thermallystable. When heated above about 180° C., the pendant carboxylic acidgroups readily convert intra-molecularly into anhydride groups, wherethe degree to which this anhydride conversion occurs is dependent on themonomer unit arrangement along the thermoplastic copolymer, as well asthe temperature to which it is heated.

As can be appreciated, during a layer transfusion step in anelectrophotography-based additive manufacturing system, the supportmaterial can be quickly heated above this temperature. As such, if thethermoplastic copolymer in the support material retains a significantnumber of carboxylic acid groups capable of converting to anhydridegroups, the layer transfusion step can undesirably initiate theanhydride conversion while printing layers of the support structure(e.g., support structure 82). Unfortunately, anhydride conversion alsoresults in the concomitant evolution of water molecules in the form ofvapor. This can result in foaming and bubbling of the heated supportmaterial during the transfusion step, potentially leading to distortionof the support layer and a failure of the overall contiguous 3D printingprocess.

As such, to avoid this issue, the thermoplastic copolymer in acid formis preferably heated to maximize (or substantially maximize) theconversion of the carboxylic acid groups into anhydride groups, therebyproviding the thermoplastic copolymer in “anhydride form”. Suitabletemperatures for heating the thermoplastic copolymer range from about160° C. to about 230° C., and more preferably from about 180° C. toabout 230° C. The heating duration may vary depending on thetemperature, such as greater than 30 minutes for a temperature of about230° C.

The thermoplastic copolymer is typically not capable of converting 100%of the carboxylic acid groups into anhydride groups upon heating, nor isthis necessary to prevent the foaming and bubbling during a layertransfusion step. Rather, only those carboxylic acid groups in monomerunits that are adjacent to each other along the backbone of thecopolymer chain are capable of intra-molecular conversion into ananhydride ring structure. In comparison, a carboxylic acid group in amonomer unit that is not adjacent to another monomer unit containing acarboxylic acid group along the backbone of the copolymer chain (i.e.,located between ethylenically-unsaturated aromatic and/or alkyl(meth)acrylate monomer units) is sterically shielded by the adjacentaromatic and/or (meth)acrylate-based ester groups, and typically cannotform anhydride structures intra-molecularly.

For instance, the ethylenically-unsaturated aromatic monomers, the alkyl(meth)acrylate monomers, and the (meth)acrylic acid monomers may bepolymerized to produce a thermoplastic copolymer in “acid form” havingthe following structure:

Upon undergoing the above-discussed heating step to convert carboxylicacid groups to anhydride groups, the copolymer illustrated in Formula 4will react to produce a thermoplastic copolymer in “anhydride form”,with concomitant evolution of water, having the following structure:

This can be further illustrated by an example in which theethylenically-unsaturated aromatic monomer is styrene, the alkyl(meth)acrylate monomer is butyl acrylate, and the (meth)acrylic acidmonomer is methacrylic acid. In this example, the thermoplasticcopolymers of Formulas 4 and 5 respectively have the followingstructures:

As illustrated by the thermoplastic copolymers in each of Formulas 5 and7, only the carboxylic acid groups in monomer units that are adjacent toeach other undergo the conversion to produce the 6-membered anhydridering structure. In comparison, the isolated carboxylic acid group ineach of Formulas 5 and 7, which is located between the(meth)acrylate-based ester group and the aromatic group, remainsunreacted.

The extent of anhydride conversion may vary depending on the particularmonomer ratios used, and more particularly, on the concentration of the(meth)acrylic acid monomer(s) relative to the total monomerconcentration. With respect to the above-discussed embodiments, themaximum amount of carboxylic acid groups that are converted to anhydridegroups typically ranges from about 60% to about 65%, relative to theinitial number of carboxylic acid groups in the copolymer prior to theanhydride conversion. In comparison, soluble thermoplastic copolymersused in support materials for extrusion-based additive manufacturingtechniques typically have substantially lower maximum anhydrideconversions, such as from about 40% to about 45%.

The term “maximum anhydride conversion” refers to a conversion in whichall carboxylic acid groups in the thermoplastic copolymer that arecapable of forming intra-molecular anhydride groups are actuallyconverted to the anhydride groups. As such, in some embodiments, thethermoplastic copolymer for the support material of the presentdisclosure has an anhydride conversion that is greater than 90%, morepreferably greater than 95%, even more preferably greater than 99%, andmost preferably 100% of the maximum anhydride conversion. For example, aconversion that is 95% of the maximum anhydride conversion, where themaximum anhydride conversion is 65% relative to the initial number ofcarboxylic acid groups, means that about 62% of the initial number ofcarboxylic acid groups are converted to anhydride groups.

Anhydride conversion is typically accompanied by a significant reductionin the glass transition temperature of the thermoplastic copolymer whencompared to the thermoplastic copolymer in acid form prior to theanhydride conversion. For example, for the above-discussed monomers, thethermoplastic copolymer in acid form may have a glass transitiontemperature ranging from about 140° C. to about 150° C., and moretypically from about 143° C. to about 148° C. In comparison, after theanhydride conversion, the same thermoplastic copolymer in anhydride formmay have a glass transition temperature ranging from about 100° C. toabout 115° C., more typically from about 105° C. to about 110° C.

This can also be seen by a reduction in the molecular weight of thecopolymer, as determined by the Molecular Weight test described below.For example, for the above-discussed monomers, the thermoplasticcopolymer in acid form may have a weight-average molecular weight (Mw)ranging from about 60,000 to about 120,000, more preferably from about65,000 to about 90,000, and even more preferably from about 70,000 toabout 75,000. In terms of number-average molecular weights (Mn), for theabove-discussed monomers, the thermoplastic copolymer in acid form mayhave a number-average molecular weight ranging from about 28,000 toabout 60,000, more preferably from about 30,000 to about 40,000, andeven more preferably from about 31,000 to about 36,000. Suitable ratiosof Mw/Mn range from about 2.00 to about 2.20.

In comparison, after the anhydride conversion, the same thermoplasticcopolymer in anhydride form may have a weight-average molecular weight(Mw) that is about 3,000 to 4,000 less. This corresponds to asignificant reduction to the thermal properties and melt viscosity ofthe thermoplastic copolymer when compared to the same thermoplasticcopolymer in acid form prior to the anhydride conversion. As discussedabove, the support material used to print support structure 82preferably has thermal properties (e.g., glass transition temperature)and a melt rheology that is similar to or substantially the same asthose of the part material used to print 3D part 80. This allows partand support materials of layers 64 p and 64 s to be heatedsimultaneously with heater 74 to substantially the same transfertemperature, and also allows the part and support materials at the topsurfaces of 3D part 80 and support structure 82 to be heated togetherwith heater 74 to substantially the same temperature. Thus, the partlayers 64 p and the support layers 64 s may be transfused together tothe top surfaces of 3D part 80 and support structure 82 in a singletransfusion step as combined layer 64.

As such, when engineering the support material of the present disclosurefor compatibility with a desired part material (e.g., an ABS partmaterial), this reduction in the thermal properties and melt rheology ofthe thermoplastic copolymer due to the anhydride conversion is accountedfor during the polymerization process to produce the thermoplasticcopolymer. In particular, as discussed further below, it has been foundthat the thermoplastic copolymer may be polymerized with molecularweight modifications to attain a desired glass transition temperatureand melt rheology profile that is similar to or substantially the sameas those of a desired part material, such the above-discussed ABS partmaterial.

Preferably, the thermoplastic copolymer of the support material (and thesupport material itself) have glass transition temperatures within about10° C. of the glass transition temperature of the part material, andmore preferably within about 5° C., where the glass transitiontemperatures are determined pursuant to the Glass Transition Temperaturetest described below. Furthermore, the thermoplastic copolymer (and thesupport material itself) preferably has dynamic viscosities at 180° C.,at 190° C., and at 200° C. that are each within about 10kilopascal-seconds of the respective dynamic viscosities for the partmaterial, more preferably within about 5 kilopascal-seconds, and evenmore preferably within about 2 kilopascal-seconds, where the dynamicviscosities are determined pursuant to the Melt Rheology test describedbelow.

For instance, when used to complement an ABS part material, such as theABS part material disclosed in co-filed U.S. patent application Ser. No.13/944,472, entitled “ABS Part Material For Electrophotography-BasedAdditive Manufacturing”, preferred glass transition temperatures for thethermoplastic copolymer (and the support material itself) range fromabout 100° C. to about 115° C., more preferably from about 105° C. toabout 110° C. In some embodiments, the glass transition temperatures forthe support material copolymer may range from about 106° C. to about108° C. Suitable dynamic viscosities for the thermoplastic copolymer(and the support material itself) at 180° C. range from about 17kilopascal-seconds to about 24 kilopascal-seconds, at 190° C. range fromabout 6 kilopascal-seconds to about 10 kilopascal-seconds, and at 200°C. range from about 3.5 kilopascal-seconds to about 4.5kilopascal-seconds.

In order to increase the commercial-viability of the manufacturingprocess, the thermoplastic copolymer is preferably isolated frompolymerization solvent (e.g., ethanol). For example, after thepolymerization process is completed, the copolymer solution may beheated to about 200° C. to about 230° C., under partial vacuum, in orderto evaporate and ensure complete removal of the solvent together withany trace amounts of residual monomers. This also provides a copolymermelt which can be extruded and pelletized for further necessary particlesize reduction techniques required to produce the copolymer powder.

Furthermore, the solvent is preferably recycled to reduce material costsfor producing the support material of the present disclosure. Forexample, after a polymerization process is completed, the solvent (e.g.,ethanol) may be distilled and recovered for use in subsequentpolymerization processes.

Furthermore, in some preferred embodiments, the solvent isolation andthe above-discussed anhydride conversion may be performed in the samestep. For example, the copolymer solution may be heated in multiplestages with increasing temperatures. The initial stage(s) (e.g., atabout 200° C. to about 220° C.) may be used to evaporate and remove thesolvent, and the later stage(s) (e.g., at about 230° C.) may be used forthe anhydride conversion. This process may also be combined withextruding the thermoplastic copolymer in strand form for subsequentpelletizing, grinding, micronization, and/or classification, such aswith a polymer isolation device commercially available under thetradename “ENTEX” Planetary Roller Extruder device from ENTEX Rust &Mitschke GmBH, Bochum, Germany.

As mentioned above, the support material is engineered for use in anEP-based additive manufacturing system (e.g., system 10) to printsupport structures (e.g., support structure 82). As such, the supportmaterial may also include one or more materials to assist in developinglayers with EP engine 12 s, to assist in transferring the developedlayers from EP engine 12 s to layer transfusion assembly 20, and toassist in transfusing the developed layers with layer transfusionassembly 20.

For example, in the electrophotographic process with system 10, thesupport material is preferably charged triboelectrically through themechanism of frictional contact charging with carrier particles atdevelopment station 58. This charging of the support material may bereferred to by its triboelectric charge-to-mass (Q/M) ratio, which maybe a positive or negative charge and has a desired magnitude. The Q/Mratio is inversely proportional to the powder density of the supportmaterial, which can be referred to by its mass per unit area (M/A)value. For a given applied development field, as the value of Q/M ratioof the support material is increased from a given value, the M/A valueof the support material decreases, and vice versa. Thus, the powderdensity for each developed layer of the part material is a function ofthe Q/M ratio of the support material.

It has been found that, in order to provide successful and reliabledevelopment of the support material onto development drum 44 andtransfer to layer transfusion assembly 20 (e.g., via belt 22), and toprint support structure 82 with a good material density, the supportmaterial preferably has a suitable Q/M ratio for the particulararchitecture of EP engine 12 s and belt 22. Furthermore, because thepart and support materials are preferably transferred together to layertransfusion assembly 20 by belt 22 (e.g., as combined layer 64), thepart and support materials preferably have similar or substantially thesame Q/M ratios.

Accordingly, the support material preferably has a Q/M ratio that is thesame sign (i.e., negative or positive) as the Q/M ratio of the partmaterial, and is also preferably within about 10 micro-Coulombs/gram(μC/g) of the Q/M ratio of the part material, more preferably withinabout 5 μC/g, and even more preferably within about 3 μC/g, where theQ/M ratios are determined pursuant to the Triboelectric Charging testdescribed below. Examples of preferred Q/M ratios for the supportmaterial range from about −5 micro-Coulombs/gram (μC/g) to about −50μC/g, more preferably from about −10 μC/g to about −40 μC/g, and evenmore preferably from about −15 μC/g to about −35 μC/g, and even morepreferably from about −25 μC/g to about −30 μC/g.

In this above-discussed embodiment, the Q/M ratio is based on a negativetriboelectric charge. However, in an alternative embodiment, system 10may operate such that the Q/M ratio of the support material has apositive triboelectric charge with the above-discussed magnitudes. Ineither embodiment, these magnitudes of Q/M ratio prevent theelectrostatic forces constraining the support material to the carriersurfaces from being too excessive, and that any level of “wrong sign”powder is minimized. This reduces inefficiencies in the development ofthe support material at EP engine 12 s, and facilitates the developmentand transfer of each layer 64 s with the desired M/A value.

Furthermore, if a consistent material density of support structure 82 isdesired, the desired Q/M ratio (and corresponding M/A value) ispreferably maintained at a stable level during an entire printingoperation with system 10. However, over extended printing operationswith system 10, development station 58 of EP engine 12 s may need to bereplenished with additional amounts of the support material. This canpresent an issue because, when introducing additional amounts of thesupport material to development station 58 for replenishment purposes,the support material is initially in an uncharged state until mixingwith the carrier particles. As such, the support material alsopreferably charges to the desired Q/M ratio at a rapid rate to maintaina continuous printing operation with system 10.

Accordingly, controlling and maintaining the Q/M ratio during initiationof the printing operation, and throughout the duration of the printingoperation, will control the resultant rate and consistency of the M/Avalue of the support material. In order to reproducibly and stablyachieve the desired Q/M ratio, and hence the desired M/A value, overextended printing operations, the support material preferably includesone or more charge control agents, which may be added to thethermoplastic copolymer during the manufacturing process of the supportmaterial. For example, the charge control agent may be melt blended withthe thermoplastic copolymer, prior to subjecting the blended materialsto grinding, micronization, and/or classification.

In embodiments in which the Q/M ratio of the support material has anegative charge, suitable charge control agents for use in the supportmaterial include acid metal complexes (e.g., oxy carboxylic acidcomplexes of chromium, zinc, and aluminum), azo metal complexes (e.g.,chromium azo complexes and iron azo complexes), mixtures thereof, andthe like.

Alternatively, in embodiments in which the Q/M ratio of the supportmaterial has a positive charge, suitable charge control agents for usein the support material include azine-based compounds, and quaternaryammonium salts, mixtures thereof, and the like. These agents areeffective at positively charging the thermoplastic copolymer whenfrictionally contact charged against appropriate carrier particles.

The charge control agents preferably constitute from about 0.1% byweight to about 5% by weight of the support material, more preferablyfrom about 0.5% by weight to about 2% by weight, and even morepreferably from about 0.75% by weight to about 1.5% by weight, based onthe entire weight of the support material. As discussed above, thesecharge control agents preferably increase the charging rate of thethermoplastic copolymer of the support material against the carrier, andstabilize the Q/M ratio over extended continuous periods of printingoperations with system 10.

In many situations, system 10 prints layers 64 s with a substantiallyconsistent material density over the duration of the printingoperations. Having a support material with a controlled and consistentQ/M ratio allows this to be achieved. However, in some situations, itmay be desirable to adjust the material density between the variouslayers 64 s in the same printing operation. For example, system 10 maybe operated to run in a grayscale manner with reduced material density,if desired, for one or more portions of support structure 82.

In addition to incorporating the charge control agents, for efficientoperation EP engine 12 s, and to ensure fast and efficient triboelectriccharging during replenishment of the support material, the mixture ofthe support material preferably exhibits good powder flow properties.This is preferred because the support material is fed into a developmentsump (e.g., a hopper) of development station 58 by auger, gravity, orother similar mechanism, where the support material undergoes mixing andfrictional contact charging with the carrier particles.

As can be appreciated, blockage or flow restrictions of the supportmaterial during the replenishment feeding can inhibit the supply of thesupport material to the carrier particles. Similarly, portions of thesupport material should not become stuck in hidden cavities indevelopment station 58. Each of these situations can alter the ratio ofthe support material to the carrier particles, which, as discussedabove, is preferably maintained at a constant level to provide thedesired Q/M ratio for the charged support material.

For example, the support material may constitute from about 1% by weightto about 30% by weight, based on a combined weight of the supportmaterial and the carrier particles, more preferably from about 5% toabout 20%, and even more preferably from about 5% to about 10%. Thecarrier particles accordingly constitute the remainder of the combinedweight.

The powder flow properties of the support material can be improved orotherwise modified with the use of one or more flow control agents, suchas inorganic oxides. Examples of suitable inorganic oxides includehydrophobic fumed inorganic oxides, such as fumed silica, fumed titania,fumed alumina, mixtures thereof, and the like, where the fumed oxidesmay be rendered hydrophobic by silane and/or siloxane-treatmentprocesses. Examples of commercially available inorganic oxides for usein the support material include those under the tradename “AEROSIL” fromEvonik Industries AG, Essen, Germany.

The flow control agents (e.g., inorganic oxides) preferably constitutefrom about 0.1% by weight to about 10% by weight of the supportmaterial, more preferably from about 0.2% by weight to about 5% byweight, and even more preferably from about 0.3% by weight to about 1.5%by weight, based on the entire weight of the support material. The flowcontrol agents may be introduced to the thermoplastic copolymer andcharge control agent at any suitable point in the manufacturing processto produce the support material. For example, the blended thermoplasticcopolymer may be further dry blended in a high speed and high shearcyclonic mixing apparatus, preferably at 25° C., with one or moreexternal flow control agents. This uniformly distributes, coats, andpartially embeds the flow control agent(s) into the individual particlesof the blended thermoplastic copolymer, without significantly alteringthe particle size or particle size distribution.

As discussed above, the one or more charge control agents are suitablefor charging the support material copolymer to a desired Q/M ratio fordeveloping layers of the support material at EP engine 12 s, and fortransferring the developed layers (e.g., layers 64) to layer transfusionassembly 20 (e.g., via belt 22). However, the multiple printed layers ina 3D environment effectively prevents the electrostatic transfer ofsupport material after a given number of layers are printed. Instead,layer transfusion assembly 20 utilizes heat and pressure to transfusethe developed layers together in the transfusion steps.

In particular, heaters 72 and/or 74 may heat layers 64 and the topsurfaces of 3D part 80 and support structure 82 to a temperature near anintended transfer temperature of the part and support material, such asat least a fusion temperature of the part and support material, prior toreaching nip roller 70. Similarly, post-fuse heater 76 is locateddownstream from nip roller 70 and upstream from air jets 78, and isconfigured to heat the transfused layers to an elevated temperature inthe post-fuse or heat-setting step.

Accordingly, the support material may also include one or more heatabsorbers configured to increase the rate at which the support materialis heated when exposed to heater 72, heater 74, and/or post-heater 76.For example, in embodiments in which heaters 72, 74, and 76 are infraredheaters, the heat absorber(s) used in the support material may be one ormore infrared (including near-infrared) wavelength absorbing materials.Absorption of infrared light causes radiationless decay of energy tooccur within the particles, which generates heat in the supportmaterial.

Suitable infrared absorbing materials for use in the support materialmay vary depending on the desired color of the support material.Examples of suitable infrared absorbing materials include carbon black(which may also function as a black pigment for the support material),as well as various classes of infrared absorbing pigments and dyes, suchas those that exhibit absorption in the wavelengths ranging from about650 nanometers (nm) to about 900 nm, those that exhibit absorption inthe wavelengths ranging from about 700 nm to about 1,050 nm, and thosethat exhibit absorption in the wavelengths ranging from about 800 nm toabout 1,200 nm. Examples of these pigments and dyes classes includeanthraquinone dyes, polycyanine dyes metal dithiolene dyes and pigments,tris aminium dyes, tetrakis aminium dyes, mixtures thereof, and thelike.

The infrared absorbing materials also preferably do not significantlyreinforce or otherwise alter the melt rheologies of the thermoplasticcopolymer of the support material, such as the zero shear viscosityversus temperature profile of the thermoplastic copolymer. For example,this can be achieved using a non-reinforcing type of carbon black, or a“low structure” type of carbon black, at low concentrations relative tothe thermoplastic copolymer. Accordingly, suitable dynamic viscositiesfor the support material include those discussed above for thethermoplastic copolymer at 180° C., 190° C., and 200° C.

In embodiments that incorporate heat absorbers, the heat absorbers(e.g., infrared absorbers) preferably constitute from about 0.5% byweight to about 10% by weight of the support material, more preferablyfrom about 1% by weight to about 5% by weight, and in some morepreferred embodiments, from about 2% by weight to about 3% by weight,based on the entire weight of the support material. The heat absorbermay be introduced to the thermoplastic copolymer at any suitable pointin the manufacturing process to produce the support material, such aswith the charge control agent. For example, the charge control agent andthe heat absorber may be melt blended with the thermoplastic copolymer,prior to subjecting the blended materials to grinding, micronization,and/or classification.

The support material may also include one or more additional additives,such as colorants (e.g., pigments and dyes in addition to, oralternatively to, the heat absorbers), polymer stabilizers (e.g.,antioxidants, light stabilizers, ultraviolet absorbers, andantiozonants), biodegradable additives, and combinations thereof. Inembodiments that incorporate additional additives, the additionaladditives may collectively constitute from about 0.1% by weight to about20% by weight of the support material, more preferably from about 0.2%by weight to about 10% by weight, and even more preferably from about0.5% by weight to about 5% by weight, based on the entire weight of thesupport material. These materials may also be introduced to thethermoplastic copolymer at any suitable point in the manufacturingprocess to produce the support material, such as during the meltblending with the charge control agent and/or heat absorber.

For use in electrophotography-based additive manufacturing systems(e.g., system 10), the support material has a controlled averageparticle size and a narrow particle size distribution, which arepreferably similar to or substantially the same as those of the partmaterial. The D50 particle sizes for the support material are preferablywithin about 15 micrometers of the D50 particle size of the partmaterial, more preferably within about 10 micrometers, and even morepreferably within about 5 micrometers, where the particle sizes andparticle size distributions are determined pursuant to the Particle Sizeand Particle Size Distribution standard described below. For example,preferred D50 particles sizes for the support material include those upto about 100 micrometers if desired, more preferably from about 10micrometers to about 30 micrometers, more preferably from about 10micrometers to about 20 micrometers, and even more preferably from about10 micrometers to about 15 micrometers.

Additionally, the particle size distributions for the support material,as specified by the parameters D90/D50 particle size distributions andD50/D10 particle size distributions, each preferably range from about1.00 to 1.40, more preferably from about 1.10 and to about 1.35, andeven more preferably from about 1.15 to about 1.25. Moreover, theparticle size distribution for the support material is preferably setsuch that the geometric standard deviation σ_(g) preferably meets thecriteria pursuant to the following Equation 1:

$\sigma_{g} \sim \frac{D\; 90}{D\; 50} \sim \frac{D\; 50}{D\; 10}$In other words, the D90/D50 particle size distributions and D50/D10particle size distributions are preferably the same value or close tothe same value, such as within about 10% of each other, and morepreferably within about 5% of each other.

The support material may be manufactured by polymerizing or otherwiseproviding the thermoplastic copolymer, melt blending the thermoplasticcopolymer with the charge control agent, and optionally with the heatabsorber and/or any additional additives, and then grinding, micronizingand/or classifying the resulting material to attain a powder having theabove-discussed particle sizes and particle size distributions. Someadditional materials, such as the flow control agent, may be blended tothe resulting powder under high shear, if desired. This uniformlydistributes, coats, and partially embeds the flow control agent(s) intothe individual particles of the support material, without significantlyaltering the particle size or particle size distribution.

It has also been found that the formulated support material, and thethermoplastic copolymer in particular, are fairly brittle. This rendersthe support material useful in powder form for electrophotography-basedadditive manufacturing systems, but less desirable for filament-basedadditive manufacturing techniques, such as the extrusion-based techniquedeveloped by Stratasys, Inc., Eden Prairie, Minn., under the trademarks“FUSED DEPOSITION MODELING” and “FDM”. It is believed that the brittlenature of the formulated support material, if provided in filament formwithout any corrective additives, will readily fracture or break whiletraveling from a consumable container to a liquefier of theextrusion-based system.

The formulated support material may then be filled into a cartridge orother suitable container for use with EP engine 12 s in system 10. Forexample, the formulated support material may be supplied in a cartridge,which may be interchangeably connected to a hopper of developmentstation 58. In this embodiment, the formulated support material may befilled into development station 58 for mixing with the carrierparticles, which may be retained in development station 58. Developmentstation 58 may also include standard toner development cartridgecomponents, such as a housing, delivery mechanism, communicationcircuit, and the like.

The carrier particles in development station 58 may be any suitablemagnetizable carrier particles for charging the support material, suchas carrier particles having strontium ferrite cores with polymercoatings. The cores are typically larger in size than the particles ofthe support material, such as averaging from about 20 micrometers toabout 25 micrometers in diameter. The polymer coatings may varydepending on the Q/M ratios desired for the support material. Examplesof suitable polymer coatings include poly(methyl methacrylate) (PMMA)for negative charging, or poly(vinylidene fluoride) (PVDF) for positivecharging. Suitable weight ratios of the support material to the carrierparticles in development station or cartridge 58 include those discussedabove.

Alternatively, development station 58 itself may be an interchangeablecartridge device that retains the supply of the support material. Infurther alternative embodiments, EP engine 12 s itself may be aninterchangeable device that retains the supply of the support material.

When the support material is loaded to system 10, system 10 may thenperform printing operations with the support material to print supportstructures (e.g., support structure 82) in coordination with 3D parts(e.g., 3D part 80). For instance, the layers 64 s of support structure82 may be developed from the support material 66 s with EP engine 12 sand transferred to layer transfusion assembly 20 along with layers 64 pof the developed part material 66 p, via belt 22. Because the part andsupport materials have similar or substantially the same Q/M ratios,belt 22 may be electrostatically attract both layers 64 p and 66 p withthe biases of the same magnitude and sign.

Upon reaching layer transfusion assembly the combined layer 64 (oflayers 64 p and 64 s) are heated and transfused to print 3D part 80 andsupport structure 82 in a layer-by-layer manner using an additivemanufacturing technique. As discussed above, because the part andsupport materials have similar or substantially the same thermalproperties and melt rheologies, the part and support materials may betransfused in a single step, rather than requiring separate anddifferent thermal transfusion steps. Additionally, the prior anhydrideconversion step during the production of the support material preventsthe support material from undesirably undergoing an additional anhydrideconversion reaction during the layer transfusion step, which canotherwise impair part quality.

Furthermore, the support material exhibits good adhesion to partmaterials, particularly with ABS part materials. This allows supportstructure 82 to anchor the layers of 3D part 80 to build platen 68,which reduces distortions and curling stresses that may apply to 3D part80 upon cooling down, and also provides good overhang support for 3Dpart 80.

Compositionally, the resulting support structure (e.g., supportstructure 82) includes the thermoplastic copolymer, the charge controlagent, and optionally, any heat absorber, flow control agent, and/or anyadditional additives. Furthermore, if desired, the transfusion stepswith layer transfusion assembly 20 may provide part densities that aregreater than those achievable from support materials with otherfusion-based additive manufacturing techniques, such as theextrusion-based technique developed by Stratasys, Inc., Eden Prairie,Minn., under the trademarks “FUSED DEPOSITION MODELING” and “FDM”.

Additionally, the thermoplastic copolymer in the support structure hassubstantially the same carboxylic acid group-to-anhydride group ratio asthat of the support material provided to system 10 (i.e., prior toprinting). As discussed above, this is due to the anhydride conversionstep that was performed during the manufacture of the support material,which preferably converted the carboxylic acid groups of thethermoplastic copolymer to anhydride groups to the fullest extentpossible. Furthermore, the support structure is soluble in an aqueoussolution, such as an aqueous alkaline solution, to provide a hands-freesupport removal process, as discussed above.

As mentioned above, the support material of the present disclosure isparticularly suitable for use with the ABS part material disclosed inco-filed U.S. patent application Ser. No. 13/944,472, entitled “ABS PartMaterial For Electrophotography-Based Additive Manufacturing”. Forexample the ABS part material may include an ABS copolymer, a chargecontrol agent, a heat absorber, and optionally, one or more additionaladditives, such as a flow control agent. The ABS copolymer preferablyincludes acrylonitrile units (derived from acrylonitrile), butadieneunits (derived from butadiene), and aromatic units (derived from anethylenically-unsaturated aromatic monomer, preferably styrene).

Collectively (prior to removal of the support structure), the 3D part(e.g., 3D part 80) printed from the ABS part material and the supportstructure (e.g., support structure 82) printed from the support materialof the present disclosure may be provided as an “object” where the 3Dpart and support structure are adhered to each other. Prior to removalof the support structure from the 3D part, the support structure maysupport overhanging features of the 3D part, significant angular slopesexist in the 3D part, areas where it is essential to also preservedelicate features in the 3D part, such as small orifices or controlledpore structures, and in some situations, to laterally encase the 3Dpart.

Property Analysis and Characterization Procedures

Various properties and characteristics of the part and support materialsdescribed herein may be evaluated by various testing procedures asdescribed below:

1. Glass Transition Temperature

The glass transition temperature is determined using the classical ASTMmethod employing Differential Scanning calorimetry (DSC) ASTM D3418-12e1and is reported in degrees Celsius. The test is performed with a DSCanalyzer commercially available under the tradename “SEIKO EXSTAR 6000”from Seiko Instruments, Inc., Tokyo, Japan, with a 10-milligram sampleof the support material copolymer. The data is analyzed using softwarecommercially available under the tradenames “DSC Measurement V 5.7” and“DSC Analysis V5.5”, also from Seiko Instruments, Inc., Tokyo, Japan.The temperature profile for the test includes (i) 25° C. to 160° C.heating rate 10 Kelvin/minute (first heating period), (ii) 160° C. to20° C. cooling rate 10 Kelvin/minute, and (iii) 20° C. to 260° C.heating rate 10 Kelvin/minute (second heating period). The glasstransition temperature is determined using only the heat flowcharacteristics of the second heating period (iii).

2. Particle Size and Particle Size Distribution

Particle sizes and particle size distributions are measured using aparticle size analyzer commercially available under the tradename“COULTER MULTISIZER II ANALYZER” from Beckman Coulter, Inc., Brea,Calif. The particle sizes are measured on a volumetric-basis based onthe D50 particles size, D10 particle size, and D90 particles sizeparameters. For example, a D50 particle size of 10.0 micrometers for asample of particles means that 50% of the particles in the sample arelarger than 10.0 micrometers, and 50% of the particles in the sample aresmaller than 10.0 micrometers. Similarly, a D10 particle size of 9.0micrometers for a sample of particles means that 10% of the particles inthe sample are smaller than 9.0 micrometers. Moreover, a D90 particlesize of 12.0 micrometers for a sample of particles means that 90% of theparticles in the sample are smaller than 12.0 micrometers.

Particle size distributions are determined based on the D90/D50distributions and the D50/D10 distributions. For example, a D50 particlesize of 10.0 micrometers, a D10 particle size of 9.0 micrometers, and aD90 particle size of 12.0 micrometers provides a D90/D50 distribution of1.2, and a D50/D10 distribution of 1.1.

As mentioned above, the geometric standard deviation σ_(g) preferablymeets the criteria pursuant to the above-shown Equation 1, where theD90/D50 distributions and D50/D10 distributions are preferably the samevalue or close to the same value. The closeness of the D90/D50distributions and D50/D10 distributions are determined by the ratio ofthe distributions. For example, a D90/D50 distribution of 1.2 and aD50/D10 distribution of 1.1 provides a ratio of 1.2/1.1=1.09, or about a9% difference.

3. Triboelectric Charging

The triboelectric or electrostatic charging properties of powder-basedmaterials for use in electrophotography-based additive manufacturingsystems, such as system 10, may be determined with the followingtechnique. A test sample of 7 parts by weight of the powder-basedmaterial is agitated in a clean dry glass bottle with 93 parts by weightof carrier particles. The carrier particles include a magnetized22-micrometer core of strontium ferrite coated with 1.25% by weight of apolymer coating of poly(methyl methacrylate) (PMMA) for negativecharging, or poly(vinylidene fluoride) (PVDF) for positive charging.

The mixture of the powder-based material and the carrier particles isagitated 25° C. on a jar roller for 45 minutes to ensure complete mixingof the carrier particles and the powder-based material, and to ensureequilibration of the Q/M ratios. This mixing simulates the mixingprocess that occurs in a development station of the electrophotographyengine when the part or support materials are added to the carrierparticles.

A sample of the mixture is then quantitatively analyzed with a TEC-3Triboelectric Charge Analyzer (available from Torrey Pines Research,Fairport, N.Y.). This analyzer uses electric fields to strip theelectrostatic powder from the carrier particle surface, and a rotatinghigh-strength, planar multi-pole magnet to constrain the (magnetizableor permanently magnetized) carrier beads to a bottom electrode.

A 0.7-gram sample of the mixture (sample powder and carrier particles)is placed onto a clean stainless steel disc, which serves as the bottomelectrode in an electrostatic plate-out experiment across a gap, underthe influence of an applied electric field. This bottom electrode ismounted and positioned above the rotating multi-pole magnet, and a cleantop plate disc electrode is mounted securely above the bottom plate, andparallel to it, so as to provide a controlled gap of 5 millimetersbetween the top and bottom electrode plates, using insulatingpolytetrafluoroethylene (PTFE under tradename “TEFLON”) spacers at theelectrodes' periphery.

If the powder is expected to charge negatively, a direct-current voltageof +1,500 volts is applied across the electrodes, and the magneticstirrer is activated to rotate at 1500 rpm, so as to gently keep thecarrier and powder under test constrained, but also slightly agitated onthe bottom electrode, during the measurement. Alternatively, if thepowder is expected to charge positively, then a negative bias voltage of−1,500 volts is applied. In either case, the applied electric fieldcauses the powder to strip from the carrier, in the powder/carriermixture, and to transfer to the top electrode, over a defined timeperiod.

The stripped powder under test is deposited on the top electrode, andthe induced accumulated charge on the top plate is measured using anelectrometer. The amount of powder transferred to the top electrode isweighed, and compared to the theoretical percentage in the originalcarrier powder mix. The carrier remains on the bottom plate due to themagnetic forces constraining it.

The total charge on the top plate and the known weight of transferredelectrostatic powder are used to calculate the Q/M ratio of the testpowder, and to also check that all the electrostatic powder hastransferred from the carrier, according to the theoretical amountoriginally mixed with the carrier beads. The time taken for completepowder transfer to the top plate, and the percent efficiency of thepowder transfer process are also measured.

4. Powder Flowability

As discussed above, the part and support materials of the presentdisclosure preferably exhibit good powder flow properties. This reducesor prevents blockage or flow restrictions of the part or supportmaterial during the replenishment feeding, which can otherwise inhibitthe supply of the part or support material to the carrier particles inthe development station. The powder flowability of a sample material isqualitatively measured by visually observing the flowability of thepowder in comparison to commercially-available toners utilized intwo-dimensional electrophotography processes, which are rated as having“good flow” or “very good flow”.

5. Melt Rheology

Preferably, the melt rheologies of the part and support materials aresubstantially the same as the melt rheologies of their respectivecopolymers, and are preferably not detrimentally affected by the otheradditives. Additionally, as discussed above, the part and supportmaterials for use with electrophotography-based additive manufacturingsystems (e.g., system 10) preferably have similar melt rheologies.

Melt rheologies of the part and support materials of the presentdisclosure, and their respective copolymers, are measured based on theirmelt flow indices over a range of temperatures. The melt flow indicesare measured using a rheometer commercially available under thetradename “SHIMADZU CFT-500D” Flowtester Capillary Rheometer fromShimadzu Corporation, Tokyo, Japan. During each test, a 2-gram sample isloaded to the rheometer pursuant to standard operation of the rheometer,and the temperature of the sample is increased to 50° C. to cause aslight compacting of the sample.

The temperature is then increased from 50° C. at a rate of 5° C. perminute, allowing the sample to first soften and then flow. The rheometermeasures the sample viscosity using the flow resistance of the melt toflow through a small die orifice, as a piston of the rheometer is driventhrough a cylinder. The rheometer records the softening point, thetemperature at which flow begins, and the rate at which flow increasesas a result of the temperature increase, until the cylinder is exhaustedof sample melt. The rheometer also calculates the apparent viscosity inPascal-seconds at each temperature point in the ramp. From this data,the apparent viscosity versus temperature profile can be determined,such as shown in FIGS. 6-8, for example.

6. Copolymer Composition

The molecular composition and the respective acid and anhydride contentof the thermoplastic copolymer of the support material is determinedusing proton nuclear magnetic resonance (NMR) spectroscopy. Thecopolymer sample preparation involves dissolving about 40 milligrams ofthe thermoplastic copolymer in 0.7 milliliters of a dimethyl sulfoxide(dmso)-d₆ solvent using ultrasonification, where the thermoplasticcopolymer in powder, pellet, or large chunk form is pulverized to fineparticles to assist in the dissolution.

Spectral data is recorded on a proton NMR spectrometer commerciallyavailable under the tradename “BRUKER AV700” Spectrometer from BrukerCorporation, Billerica, Mass., which is equipped with a cryoprobe. Theproton NMR spectrum is recorded at 700.23 megahertz, and sampletemperature is maintained at 25° C. using a Bruker digital variabletemperature unit.

The analysis is performed by initially cleaning the sample to remove anysignificant amounts of other aromatic species or residual monomers. Thecleaning involves washing or re-suspending in the thermoplasticcopolymer sample in heptane or hexane, followed by filtration and dryingunder vacuum at 75° C. This facilitates accurate integrations of the keysignals in the spectrum.

Two preliminary scans are then conducted on the cleaned sample with theproton NMR spectrometer, which allows a steady state to be attained,followed by 32 co-added 30-degree pulse transients, resulting in a goodsignal-to-noise ratio. Each pulse acquisition time is 2.94 seconds, witha delay of 2.0 seconds between pulses, for a total recycle time of 4.94seconds. The data is processed using zero-filling and a 0.2 hertz linebroadening apodization function. The spectral width is 16.0parts-per-million (ppm), spanning −2.0 ppm to 14.0 ppm. The spectrum isreferenced to residual protons in the deuterated solvent, which occur asa quintet at 2.50 ppm for the dmso-d₆ solvent. Integrals are relative toeach other, on a molar basis.

FIG. 5 illustrates an example proton NMR spectrum for a thermoplasticcopolymer polymerized from styrene, n-butyl acrylate, and methacrylicacid, in an ethanol solution polymerization, where the peak at about 12ppm is due to the single proton of the carboxylic acid group from themethacrylic acid, the peak at about 7 ppm is due to the five protons ofthe aromatic ring from the styrene monomer, and the peak at about 3.5ppm to 4.0 ppm, which is typically well resolved in the spectrum, is dueto the two protons of the methylene group adjacent to oxygen atom in theester group from the n-butyl acrylate.

It is useful to deliberately set the aromatic ring peak to integrate to5.0 protons as a reference. Integration of these three areas allows themolar ratio of the three monomers to be calculated, and assumes that allof the styrene monomers (or other aromatic monomers) have beenincorporated into the polymer. Thus, it is possible to calculate thecopolymer composition, prior to anhydride conversion, relative to themolar ratio of the starting theoretical monomer composition. Likewise,from this spectrum, and comparison to the spectrum of any anhydrideconverted material, the integration of the peak at about 12 ppm willdecrease relative to aromatic ring peak at about 7.0 ppm, where thesignal for the aromatic ring peak at 7 ppm is again deliberately set tointegrate to 5.0 protons. From this, the percent conversion of thecarboxylic acid groups to anhydride groups can be calculated. Providedthat the copolymer is not heated above about 260° C. during theanhydride-conversion step, no decomposition of the copolymer or loss ofmonomers is typically observed.

7. Molecular Weight

The molecular weight of the support material copolymer is determinedusing a Gel Permeation Chromatography (GPC) technique with a GPCinstrument that includes a high-performance liquid chromatography (HPLC)solvent pump, fully automated sampling equipment, separation columns,and refractive index (RI)-detector.

The copolymer sample is prepared by dissolving 10-15 milligrams of thethermoplastic copolymer in 3 milliliters of a tetrahydrofuran (THF)solvent that contains 0.1% by weight of benzophenone. The copolymersolution is then filtered through a 0.45 micron filter, and run throughseparation columns of the GPC spectrometer with HPLC-grade THF solventwith 0.1% by volume of trifluoroacetic acid added. The trifluoroaceticacid prevents interaction and bonding of carboxylic acid groups of thethermoplastic copolymer with the column packing, which can otherwisecause discrepancies in elution times.

The three separation columns include: (i) a 50 millimeter (mm)×13 mmcolumn packed with a 5-micrometer styrene-divinylbenzene (SDV) copolymernetwork, (ii) a 300 mm×13 mm column packed with a 102-Angstrom SDVcopolymer network, and (iii) a 300 mm×13 mm column packed with a104-Angstrom SDV copolymer network. GPC data is evaluated relative toknown polystyrene standards with elution time correction usingbenzophenone as an internal standard. The data is analyzed usingsoftware commercially available under the tradename “WIN GPC V 6.10”from Polymer Standards Service-USA, Inc., Amherst, Mass.

8. Acid Value

The acid value or total acid content of the thermoplastic copolymer (thesum of acid and anhydride groups) is determined by titration as follows:A 300 milligram sample of the copolymer is dissolved in a solution of 30milliliters of analytical-grade ethanol, and 10.0 milliliters of 0.1 N(normality) sodium hydroxide (NaOH). The test is performed using apotentiometric titration procedure with 0.1 N hydrochloric acid (HCl)using a titration system commercially available under the tradename“TITROLINE ALPHA PLUS” with a pH-sensitive electrode commerciallyavailable under the tradename “H 62”, both from Schott AG, Mainz,Germany.

Each tested sample is analyzed by a fully automated dynamic titrationprocedure, and is tested in duplicate. The data is evaluated usingsoftware commercially available under the tradename “TITRISOFT” Vers.2.5 software, also from Schott AG, Mainz, Germany. It is noted that thisis a “back-titration” after conversion of all original acidic functionsinto sodium salts, using excess sodium hydroxide. The initially clearcopolymer solution becomes slightly turbid during the addition of theHCl solution, which is observable at pH's less than about 6-8. However,the evaluation of the signal (a derivative of pH-change with the volumeof HCl added) is unaffected by this precipitation effect.

EXAMPLES

The present disclosure is more particularly described in the followingexamples that are intended as illustrations only, since numerousmodifications and variations within the scope of the present disclosurewill be apparent to those skilled in the art.

1. Example 1

The thermoplastic copolymers of Examples 1-6 were each prepared usingthe same technique, where the amounts of the polymerization initiatorwere varied to specifically modify the molecular weights of thecopolymers. For each thermoplastic copolymer, the process involvedcharging 17.0 grams of styrene (34% by weight of monomers), 12.5 gramsof n-butyl acrylate (25% by weight of monomers), and 20.5 grams ofmethacrylic acid (41% by weight of monomers), along with 35.0 grams ofethanol, to a 250-milliliter, 3-neck flask, equipped with inlet tubingfor a nitrogen gas line, a reflux condenser, and a mechanical stirrer.The monomers were each commercially available under the tradename “ACROSORGANICS” from Fischer Scientific, Inc., Pittsburgh, Pa.

The resulting monomer solution was purged with nitrogen for 30 minutesby bubbling the nitrogen gas through the liquid, and a slow flow ofnitrogen was then maintained during the whole polymerization procedure.The flask was then placed in a preheated oil bath maintained at 82° C.,and after a 10 minute period, a first addition of a polymerizationinitiator was added to the reaction mixture. The polymerizationinitiator was a 40% suspension of dibenzoyl peroxide in watercommercially available under the tradename “PERKADOX L-W40” from AkzoNobel N.V., Amsterdam, Netherlands.

The resulting polymerization reaction was then stirred for two hours at82° C. A viscous product was formed after that period. Then, a secondaddition of the polymerization initiator was added to the reactionmixture. The reaction mixture was then stirred for further two hours at82° C., followed by a third addition of the polymerization initiator.The polymerization was then allowed to continue for a further four hoursat 82° C.

Table 1 lists the amounts of the dibenzoyl peroxide polymerizationinitiator that were added to the monomers during each of the first,second, and third additions, where the addition of initiator in threeseparate steps was performed to avoid or mitigate any excessive exothermfrom occurring during the polymerization reaction, and to ensure thatthe polymerization proceeded to completion in each case.

TABLE 1 Initiator - Initiator - Initiator - Initiator first second thirdTotal Example addition (mg) addition (mg) addition (mg) (mg) Example 1450 172 172 794 Example 2 585 223 172 980 Example 3 676 256 172 1104Example 4 1012 386 172 1570 Example 5 1520 580 172 2272 Example 6 1012386 172 1570

After the polymerization was completed, the resulting viscous polymersolution was diluted with 35 grams of ethanol and allowed to cool toroom temperature. Next, the copolymer solution in ethanol wasprecipitated into 1.5 liters of cyclohexane under vigorous stirring. Asticky material was obtained that separated to the bottom. The excesscyclohexane was then decanted and the resulting thermoplastic copolymerwas stirred twice with 500 milliliters of cyclohexane for two hoursusing a mechanical stirrer.

The solvent was then separated and the sticky thermoplastic copolymerwas dried under vacuum at 100° C. overnight. A glassy thermoplasticcopolymer was then recovered and ground into powder form. The resultantpowder was dried at 100° C. under vacuum until a constant weight wasobtained. An analysis of the composition of the thermoplastic copolymersusing the Copolymer Composition Test described above showed that eachthermoplastic copolymer of Examples 1-6 was in acid form (i.e., noanhydride groups), and had monomer unit concentrations as shown below inTable 2.

TABLE 2 Styrene (percent Butyl acrylate Methacrylic acid Example byweight) (percent by weight) (percent by weight) Example 1 36.0 24.0 40.0Example 2 35.5 24.5 40.0 Example 3 34.0 25.0 41.0 Example 4 34.5 24.541.0 Example 5 34.0 25.0 41.0 Example 6 34.0 25.0 41.0

The molecular weight was also measured pursuant to the Molecular Weighttest described above. Table 3 lists the resulting weight-averagemolecular weights (Mw), the number-average molecular weights (Mn), andthe ratios thereof (Mw/Mn) for the thermoplastic copolymers of Examples1-6 in acid form.

TABLE 3 Molecular Molecular Example weight (Mw) weight (Mn) Mw/MnExample 1 115,000 56,030 2.05 Example 2 95,000 47,000 2.02 Example 383,000 38,020 2.18 Example 4 71,390 32,752 2.18 Example 5 60,000 29,0002.07 Example 6 71,577 35,560 2.01

The percent yield and glass transition temperature of each thermoplasticcopolymer of Examples 1-6 in acid form was also measured, where thepercent yields were each based on the weight of the thermoplasticcopolymer relative to the total weight of monomers used in thepolymerization reaction. The glass transition temperatures were measuredpursuant to the Glass Transition Temperature test described above. Table4 lists the percent yield and glass transition temperature of eachthermoplastic copolymer of Examples 1-6 in acid form.

TABLE 4 Percent Glass transition Example yield temperature (° C.)Example 1  97% 148 Example 2 >98% 145 Example 3 >98% 145 Example 4 >98%143 Example 5  97% 144 Example 6 >98% 144

Each thermoplastic copolymer was then subjected to an anhydrideconversion step, which involved heating the thermoplastic copolymer to230° C. for 30 minutes in an air circulating oven. The extent of theanhydride conversion was analyzed using the Copolymer Composition Testdescribed above. For each thermoplastic copolymer, it was found that nofurther anhydride conversion occurred after heating for 30 minutes.

Additionally, the acid values for the thermoplastic copolymers ofExamples 1-6 (sum of acid and anhydride groups) were determined pursuantto the Acid Value test described above. Moreover, the glass transitiontemperatures of the thermoplastic copolymers of Examples 1-6 inanhydride form were measured pursuant to the Glass TransitionTemperature test described above. Table 5 lists the percent anhydrideconversion, the acid values, and the glass transition temperatures (inanhydride form) for the copolymers of Examples 1-6.

TABLE 5 Glass transition Percent anhydride Acid temperature (° C.)Example conversion Value (anhydride form) Example 1 65% 40.8 115 Example2 63% 40.5 113 Example 3 61% 41 110 Example 4 64% 41.2 107 Example 5 64%40.9 105 Example 6 65% 41.5 108

The percent anhydride conversions shown in Table 5 corresponded to the“maximum anhydride conversions” that the thermoplastic copolymers werecapable of achieving. Additionally, a comparison of the glass transitiontemperatures shown in Table 4 (i.e., acid form) and the glass transitiontemperatures shown in Table 5 (i.e., anhydride form) illustrates how theanhydride conversion process lowers the glass transition temperature ofthe resulting thermoplastic copolymer. As discussed above, this isbelieved to correspond to a change in the molecular weight and meltrheology of the thermoplastic copolymer.

The thermoplastic copolymers of Examples 1-6 were also tested pursuantto the Melt Rheology test described above to determine how molecularweight and anhydride conversion affected the melt rheologies of thecopolymers. FIG. 6 is a plot of the resulting dynamic viscosities versustemperature for the tests, where a sample of the thermoplastic copolymerof Example 1 in acid form (i.e., prior to anhydride conversion) was alsotested for comparison purposes. Furthermore an ABS part material having1% by weight of a charge control agent, and 2.5% of a carbon black heatabsorber, as described in Example 4 of co-filed U.S. patent applicationSer. No. 13/944,472, entitled “ABS Part Material ForElectrophotography-Based Additive Manufacturing”, was also tested.

As shown in FIG. 6, the anhydride conversion step reduced the dynamicviscosity of the thermoplastic copolymer of Example 1. However, the meltrheology of the thermoplastic copolymer of Example 1 in anhydride formwas still dissimilar from that of the ABS part material tested. Incomparison, the thermoplastic copolymers of Examples 4 and 6 inanhydride form exhibited the best matches to the ABS part material formelt rheologies. Thus, this defines the target molecular weight andcopolymer composition that is the most complimentary support materialcopolymer for printing with the ABS part material in anelectrophotography-based additive manufacturing system (e.g., system10). The glass transition temperatures of the thermoplastic copolymersof Examples 4 and 6, in their thermally-stable anhydride forms, werealso good matches for the ABS part material (i.e., ranging from about106-108° C.).

2. Examples 7 and 8

The above-discussed thermoplastic copolymers of Examples 1-6 wereproduced in small scale batches for purposes of material testing andprinting with an electrophotography-based additive manufacturing system.In comparison, the thermoplastic copolymers of Examples 7 and 8 wereeach produced in a larger-scale manner to demonstrate that the supportmaterial of the present disclosure can be manufactured in acost-effective manner. The process was performed with the use of apolymer isolation device commercially available under the tradename“ENTEX” Planetary Roller Extruder (PRE) device from ENTEX Rust &Mitschke GmBH, Bochum, Germany. It was found that the PRE device waseffective in the removal of the ethanol from the copolymer solution,upon completion of the polymerization. In addition, it was found thatthe PRE device was capable of performing the maximum anhydrideconversion simultaneously with this solvent removal step.

The process for producing the thermoplastic copolymers of Examples 7 and8 initially involved preconditioning and filling a polymerization vessel(separate from the PRE device). This involved purging the polymerizationvessel with nitrogen for 30 minutes, and then charging thepolymerization vessel under a slow stream of nitrogen with 122.4kilograms of styrene (34% by weight of monomers), 90.0 kilograms ofn-butyl acrylate (25% by weight of monomers), and 147.6 kilograms ofmethacrylic acid (41% by weight of monomers), as well as 240.0 kilogramsof ethanol (containing 1% by weight of methyl ethyl ketone (MEK)). Themonomers were each commercially available under the tradename “ACROSORGANICS” from Fischer Scientific, Inc., Pittsburgh, Pa. Stirring wasthen commenced in the polymerization vessel at a rate of 60-80revolutions-per-minute (rpm).

The stirred polymerization vessel was then heated until a slight boilingof the monomer solution was observed (internal temperature of about85-86° C., after a time period of 1.5-2 hours), and an emergency stopprocedure was in place in the event that the internal temperaturereached 90° C. (reaction product would be discarded).

The monomer solution was then stirred for 30 minutes at 85-86° C., andthen cooled down to 82° C. A first addition of 3.52 kilograms of thepolymerization initiator (a 40% suspension of dibenzoyl peroxide inwater commercially available under the tradename “PERKADOX L-W40” fromAkzo Nobel N.V., Amsterdam, Netherlands) was then charged to thepolymerization vessel, along with 0.25 liters of rinsing water. Themonomer solution was then heated to 86° C. and stirred untilpolymerization reaction started, which resulted in moderate refluxing ofthe solvent at the condenser (after about 20 to 40 minutes). Whensolvent refluxing was observed (corresponding to the start of thepolymerization reaction), the internal temperature for thepolymerization vessel was set to 80° C., and stirring continued for twohours. An exothermic effect was slightly noticeable, which fell off overthe two-hour period.

A second addition of 1.09 kilograms of the polymerization initiator wasthen charged to the polymerization vessel, along with 0.25 liters ofrinsing water. The polymerization vessel was then stirred for anadditional two hours. During this period, a slight exothermic effect wasobserved.

A third addition of 1.09 kilograms of the polymerization initiator wasthen charged to the polymerization vessel, along with 0.25 liters ofrinsing water. The polymerization vessel was then stirred for anadditional two hours, during which almost no exothermic effect wasobserved.

After the polymerization reaction was completed, the resultingthermoplastic copolymer was provided in a viscous copolymer solution. A60-kilogram supply of ethanol was then charged to the polymerizationvessel to dilute the copolymer solution. The internal temperature waslowered to 70° C., and the polymerization vessel was then stirred for anadditional 1.5 hours. After this period, stirring was continued and thenitrogen flow into the polymerization vessel was stopped, and thepolymerization vessel was pressurized with nitrogen to about 2.5-3.0bars for removal of the copolymer solution.

An analysis of the composition of the thermoplastic copolymers using theCopolymer Composition Test described above showed that eachthermoplastic copolymer of Examples 7 and 8 was in acid form (i.e., noanhydride groups), and had monomer unit concentrations as shown below inTable 6.

TABLE 6 Styrene (percent Butyl acrylate Methacrylic acid Example byweight) (percent by weight) (percent by weight) Example 7 35.0 25.0 40.0Example 8 35.5 23.5 41

The percent yield and glass transition temperature of each thermoplasticcopolymer of Examples 7 and 8 in acid form were also measured, where thepercent yields were each based on the weight of the thermoplasticcopolymer relative to the total weight of monomers used in thepolymerization reaction. The glass transition temperatures were measuredpursuant to the Glass Transition Temperature test described above. Table7 lists the percent yield and glass transition temperature of eachcopolymer of Examples 7 and 8 in acid form.

TABLE 7 Percent Glass transition Example yield temperature (° C.)Example 7  98% 145 Example 8 >98% 145

The copolymer solution was then fed from the polymerization vessel tothe PRE device using a gear pump and heated tubing at a pump rate ofabout 50 liters/hour. Prior to feeding the copolymer solution, the PREdevice was set up to operate with a central spindle speed of 285 rpm, aset temperature for segment 1 and 2 of 190° C., a set temperature forsegment 3 and 4 of 230° C., a set temperature for segment 5 and 6 of220° C., and a set temperature for the die nozzle plate of 240° C.

The copolymer solution fed to the PRE device was extruded initially as afoamy brownish material on the nozzle. This copolymer fraction wasdiscarded. As soon as almost white copolymer foam was observed at thenozzle (due to entrained bubbles), the segments 3 and 4 were adjustedgradually to a vacuum of 200 to 300 millibars. This changed theextrudate into an almost colorless strand, substantially free ofbubbles. After a 15-minute period starting when the almost-clear andcolorless strand was observed, the extrudate strand was set on astainless steel, cooling belt and transported to a pelletizer. Thepelletizer according cut the received stand into pellets, which weresealed in a bag to exclude moisture.

As mentioned above, it was found that the PRE was effective in theremoval of the ethanol from the copolymer solution, upon completion ofthe polymerization. When the copolymer solution was fed to the PREdevice to extrude the stand, the ethanol was distilled off via a refluxcondenser and returned to storage tanks for further use. As such, theethanol was recyclable for use in subsequent polymerization reactions toproduce additional batches of the support material copolymer.

As also mentioned above, it was found that the PRE device was capable ofperforming the anhydride conversion step simultaneously with the ethanolremoval step. This was confirmed by analysis of the composition of thethermoplastic copolymer using the Copolymer Composition Test describedabove, which confirmed that the maximum anhydride conversion for eachthermoplastic copolymer was achieved.

The acid values for the thermoplastic copolymers of Examples 7 and 8(sum of acid and anhydride groups) were also determined pursuant to theAcid Value test described above, and the glass transition temperaturesof the thermoplastic copolymers of Examples 7 and 8 in anhydride formwere also measured pursuant to the Glass Transition Temperature testdescribed above. Table 8 lists the percent anhydride conversion, theacid values, and the glass transition temperatures for the copolymers ofExamples 7 and 8.

TABLE 8 Percent anhydride Acid Glass transition Example conversion valuetemperature (° C.) Example 7 63% 41.0 109 Example 8 62% 42.3 110

Accordingly, these results confirmed that the polymerization andisolation of the thermoplastic copolymer, with required anhydrideconversion, can be performed on a manufacturing scale, with reproducibleresults to achieve a cost-effective process.

The thermoplastic copolymers of Examples 6-8 were further testedpursuant to the Melt Rheology test described above to determine howmolecular weight and anhydride conversion affected the melt rheologiesof the copolymers. FIG. 7 is a plot of the resulting dynamic viscositiesversus temperature for the tests, where the above-discussed ABS partmaterial was also tested.

As shown in FIG. 7, the thermoplastic copolymers of Examples 7 and 8(each in anhydride form) exhibited good matches to the ABS part materialfor thermal properties and melt rheologies. Thus, the larger-scalemanufacturing was suitable for producing the thermoplastic copolymersfor use as a support material for the ABS part material. This is inaddition to the cost-effective process attainable with the PRE device.

3. Examples 9-12

The thermoplastic copolymer of Example 7 was also blended with a carbonblack heat absorber as an enhancement for fusing the powders during alayer transfusion step in an electrophotography-based additivemanufacturing system. Accordingly, support materials of Examples 9-12included the thermoplastic copolymer of Example 7 and 2.5% by weight ofcarbon black, which was commercially available under the tradename“REGAL 330” from Cabot Corporation, Boston, Mass. The carbon black wasmelt-blended by twin screw extrusion into the thermoplastic copolymer,at several different temperatures, namely 160° C. for Example 9, 180° C.for Example 10, 200° C. for Example 11, and 230° C. for Example 12. Itwas also found that no degradation of the molecular weight of thethermoplastic copolymer occurred as a result of the extrusionmelt-blending of carbon black into thermoplastic copolymer.

The support materials of Examples 7-12 were measured pursuant to theMelt Rheology test described above to determine how molecular weight andanhydride conversion affected the melt rheologies of the supportmaterials. FIG. 8 is a plot of the resulting dynamic viscosities versustemperature for the tests, where the above-discussed ABS part materialwas also tested.

As shown in FIG. 8, the incorporation of the carbon black (2.5% byweight) did not have any significant detrimental effects on the meltrheology of the support material, and the support materials of Examples8-12 continued to exhibit good matches to the ABS part material forthermal properties and melt rheologies. As such, the inclusion of thecarbon black at concentrations shown to be effective for use inelectrophotography-based additive manufacturing systems allow theresulting support materials to have melt rheologies that aresubstantially the same as the melt rheologies of the thermoplasticcopolymers of the support material, as well as the melt rheologies ofthe ABS part material.

4. Examples 13-15

The thermoplastic copolymer of Example 7 was also subjected to grindingand micronization to attain a desired particle size and particle sizedistribution for use in an electrophotography-based additivemanufacturing system. This involved grinding the thermoplastic copolymerof Example 7 to about a 300-micrometer size using a granulatorcommercially available Cumberland, Inc., New Berlin, Wis. For Example13, the pre-grind of the thermoplastic copolymer was used in neat form(i.e., unblended).

For Example 14, the pre-grind of the thermoplastic copolymer was blendedwith 2.5% by weight of carbon black in a 40-liter “HENSCHEL” blender(commercially available from Zeppelin Reimelt GmbH, Kassel, Germany),where the carbon black was commercially available under the tradename“REGAL 330” from Cabot Corporation, Boston, Mass. This blend was thenfed to a twin-screw extruder for melt-compounding. The averagetemperatures achieved during the melt compounding at steady stateconditions were: zone 1 at 164° C., zone 2 at 164° C., zone 3 at 166°C., zone 4 at 182° C., and a die temperature at 177° C. The extrudatewas then ground to about 250 micrometers and subjected to fine screeningto remove large particles.

For Example 15, the pre-grind of the thermoplastic copolymer wasblended, extruded, ground, and screened in the same manner as forExample, 14, where the blend included the thermoplastic copolymer, 2.5%by weight of the carbon black, and 1.0% by weight of a charge controlagent (zinc complex of di-t-butyl salicylate). Table 9 lists theconcentrations of the materials for Examples 13-15.

TABLE 9 Copolymer Carbon Black Charge Control (percent (percent Agent(percent Example by weight) by weight) by weight) Example 13 100.0 0.00.0 Example 14 97.5 2.5 0.0 Example 15 96.5 2.5 1.0

Each support material was then subjected to micronization by air-jetmilling and classification in an Alpine Jet Mill (Model No. 100 AFG),which was configured in tandem with an Alpine “TURBOPLEX” ATP AirClassifier, each commercially available from Hosokawa Micron Ltd.,Cheshire, England. The air classifier had a multi-wheel design,facilitating classification of the powder as a function of theperipheral speed, or the speed of the classifying wheel and the radialspeed of the air flowing through the classifying wheel. The powder'sfineness was controllable by altering the speed of the classifier wheel.Rejected material was discharged at the bottom of the classifier, andoversized particles were returned to the air-jet mill. Fines wereremoved to an air bag.

Each support material was run at a steady state grinding rate of 0.5kilograms per hour for 40 hours. The resulting powder was subjected totwo passes of the classifier in order to achieve the specifiedtolerances in particle size distribution. The optimum speed of theclassifier wheel was established to be 10,000 rpm in order to deliverthe particle size specifications (reported on a volume basis) as shownin Table 10.

TABLE 10 Percent yield Example D10 D50 D90 D50/D10 D90/D10 (by weight)Target 10.0 12.0 14.4 1.2 1.2 — Specification Example 13 10.1 12.2 14.41.2 1.2 42% Example 14 10.2 12.3 14.4 1.2 1.2 55% Example 15 10.3 12.514.7 1.2 1.2 62%

As shown in Table 10, the materials of Examples 13-15 each exhibitedparticle sizes and particle size distributions within specification andexperimental error. In addition, as illustrated in FIG. 9, the particlesizes and particle size distributions were almost identical to those ofthe ABS part material. Melt viscosity versus temperature profiles werealso measured for each material of Examples 13-15, and each was found tobe identical within experimental error to the melt rheological curvesdescribed above for the copolymer of Example 11 (containing 2.5% byweight carbon black, and melt processed at 200° C.).

7. Example 16

The classified support material powder from Example 15, containing thethermoplastic copolymer, the carbon black heat absorber, and the chargecontrol agent, was then surface treated with a flow control agent. Thisinvolved charging 400 grams of the classified powder from Example 15into a stainless steel mini “HENSCHEL” blender (commercially availablefrom Zeppelin Reimelt GmbH, Kassel, Germany), along with 2 grams of adimethyldichlorosilane-treated fumed silica commercially available underthe tradename “AEROSIL R972” from Evonik Industries AG, Essen, Germany.The mixture was then blended for several 30-second on/30-second off,high-shear, high-speed blending cycles. The resultant support materialof Example 16 exhibited significantly better powder flow properties whencompared to the original powder of Example 15. Additionally, theparticle size and size distribution were identical to the powder fromExample 15.

8. Triboelectric Charging Testing for Examples 13-16

The support materials of Examples 13-16 were subjected to triboelectriccharging analysis pursuant to the Triboelectric Charging test describedabove. Each sample was tested with carrier particles having PMMAcoatings, which provided negative charges. Table 11 lists the results ofthe triboelectric charging tests for the support materials of Examples13-16.

TABLE 11 Carrier Particles Q/M Ratio Transit Time Transit ExampleCoating (μC/g) (seconds) Efficiency Example 13 PMMA −38 ± 1 >420 95%Example 14 PMMA −18 ± 1 150 95% Example 15 PMMA −30 ± 1 120 98% Example16 PMMA −32 ± 1 45 98%

As shown in Table 11, the Q/M ratios of the support materials weredependent on the types of carrier particles used. Furthermore, thefastest transit times and greatest powder transit efficiencies wereachieved using a combination of a charge control agent and carbon blackas internal additives, and the flow control agent as a powder flowsurface additive (i.e., Example 16).

9. Printing Runs for Example 16

The support material of Example 16 was also used to print supportstructures in coordination with 3D parts printed from an ABS partmaterial, using an electrophotography-based additive manufacturingsystem corresponding to system 10 (without heater 74). The ABS partmaterial included an ABS copolymer, 1% by weight of the charge controlagent, 2.5% by weight of the carbon black heat absorber, and 0.5% byweight of the flow control agent, as described in Example 5 of co-filedU.S. patent application Ser. No. 13/944,472, entitled “ABS Part MaterialFor Electrophotography-Based Additive Manufacturing”.

During a given printing run, a digital model of a 3D part was slicedinto multiple layers, and support layers were then generated to supportoverhanging regions of the 3D part. Printing information for the slicedlayers was then transmitted to the electrophotography-based additivemanufacturing system, which was then operated to print the 3D part.

During the printing run, the ABS part material and the support materialwere each charged and developed in multiple successive layers with an EPengine of the system, where the development drums was each charged at−500 volts. The charge control agents and the flow control agents weresufficient to develop the layers with good material density. Thedeveloped layers were then transferred to an intermediary drums chargedat +450 volts, and were then transferred to a transfer belt of thesystem with biasing rollers charged at +2,000 volts. The part andsupport material layers were then transferred together to the layertransfusion assembly of the system, where the Q/M ratios of the part andsupport materials were also sufficient to maintain electrostaticattraction of the developed layers to the belt.

At the pre-heater (corresponding to heater 72), each layer was heated byinfrared radiation to temperatures ranging from about 180° C. to about200° C. The heated layers were then pressed between the nip roller andthe reciprocating build platen (with the previously-printed layers ofthe 3D part), where the nip roller was maintained at a temperature of200° C., and an average nip pressure of about 40 pounds/square-inch(psi). Each layer successfully transferred from the belt and remainedadhered to the top surface of the 3D part/support structure. Afterpassing the nip roller, the top surface of the 3D part/support structurewas then heated with a post-heater (corresponding to post-heater 76) tofurther transfuse the layers, and then cooled down with air jets. Thisprocess was then repeated for each layer of the 3D part/supportstructure.

After the printing run was completed, the 3D part/support structure wasremoved from the system and exhibited good part resolutions upon visualinspection. The 3D part/support structure was then placed in a supportremoval system commercially available under the tradename “WAVEWASH”from Stratasys, Inc., Eden Prairie, Minn. The support removal systemsubjected the combined 3D part/support structure to an aqueous alkalinesolution under agitation for a standard operating duration. Uponcompletion, the support structure (from the support material of Example16) was dissolved away from the 3D part of the ABS part material.

Accordingly, the electrophotography-based additive manufacturing systemsuccessfully printed 3D parts and support structures from the ABS partmaterial and the support material of Example 16. This is believed to bedue in part to the nearly identical melt viscosity versus temperatureprofiles, nearly identical glass transition temperatures, and nearlyidentical triboelectric charging properties of the part and supportmaterials. Furthermore, the layers were developed and transfused at fastprinting rates, with good adhesion, allowing the 3D parts and supportstructures to be printed with short printing durations and thin layers.

Although the present disclosure has been described with reference topreferred embodiments, workers skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the disclosure.

The invention claimed is:
 1. A support material comprising: a watersoluble thermoplastic copolymer in powder form; a charge control agent;and wherein the support material is configured for use in anelectrophotography-based additive manufacturing system having a layertransfusion assembly for printing the support material in alayer-by-layer manner in coordination with printing a three-dimensionalpart.
 2. The support material of claim 1, wherein the copolymercomprises aromatic groups, (meth)acrylate-based ester groups, carboxylicacid groups, and anhydride groups.
 3. The support material of claim 1,wherein the charge control agent is selected from the group consistingof chromium oxy carboxylic acid complexes, zinc oxy carboxylic acidcomplexes, aluminum oxy carboxylic acid complexes, and mixtures thereof.4. The support material of claim 1 configured to maintain a stabletriboelectric charge.
 5. The support material of claim 1, wherein thethermoplastic copolymer has an anhydride conversion that is at least 90%of a maximum anhydride conversion for the thermoplastic copolymer. 6.The support material of claim 5, wherein the copolymer is configured totransfuse to a previous layer of support material when the copolymer isheated and two or more layers of support material are pressed together.7. The support material of claim 1, wherein the thermoplastic copolymeris polymerized from monomers comprising an ethylenically-unsaturatedaromatic monomer, an alkyl (meth)acrylate, and a (meth)acrylic acid. 8.The support material of claim 1, and further comprising a flow controlagent.
 9. A method for printing a support structure with anelectrophotography-based additive manufacturing system, the methodcomprising: providing a support material in powder form comprising awater soluble thermoplastic copolymer and a charge control agent to theelectrophotography-based additive manufacturing system; electricallycharging the support material; and developing layers of the supportstructure from the charged support material with an electrophotographyengine.
 10. The method of claim 9, and further comprising:electrostatically attracting the developed layers from theelectrophotography engine to the transfer medium; moving the attractedlayers to the layer transfusion assembly with the transfer medium; andtransfusing the moved layers to previously-printed layers of the supportstructure with the layer transfusion assembly.
 11. The method of claim9, wherein the support material comprises a thermoplastic copolymerpolymerized from monomers comprising an ethylenically-unsaturatedaromatic monomer, an alkyl (meth)acrylate, and a (meth)acrylic acid. 12.The method of claim 11, wherein the ethylenically-unsaturated aromaticmonomer comprises styrene, the alkyl (meth)acrylate comprises n-butylacrylate, and the (meth)acrylic acid comprises methacrylic acid.
 13. Themethod of claim 9, wherein the powder form of the support material has aD50 particle size ranging from about 5 micrometers to about 30micrometers, and a D90/D50 particle size distribution and a D50/D10particle size distribution each ranging from about 1.00 to about 1.40.14. The method of claim 9, wherein the thermoplastic copolymer hasaromatic groups, (meth)acrylate-based ester groups, carboxylic acidgroups, and anhydride groups, the thermoplastic copolymer having ananhydride conversion that is at least 90% of a maximum anhydrideconversion for the thermoplastic copolymer.
 15. The method of claim 9,wherein electrically charging the support material comprisestriboelectrically charging the support material to a Q/M ratio having anegative charge or a positive charge, and a magnitude ranging from about5 micro-Coulombs/gram to about 50 micro-Coulombs/gram.
 16. A supportmaterial comprising: a water soluble thermoplastic copolymer in powderform; a charge control agent constituting from about 0.1% by weight toabout 5% by weight of the part material; and wherein the supportmaterial is configured for use in an electrophotography-based additivemanufacturing system having a layer transfusion assembly for printingthe support material in a layer-by-layer manner in coordination withprinting a three-dimensional part.
 17. The support material of claim 16,and further comprising a heat absorber constituting from about 0.5% byweight to about 10% by weight of the part material.
 18. The partmaterial of claim 16, wherein the copolymer is anacrylonitrile-butadiene-styrene (ABS) copolymer.